Even though they are two of the Internet’s largest advertising businesses, both Yahoo and Microsoft are without top execs to lead those units.

Worse, both are just entering a complex online ad sales and search partnership together, which will require a lot of management firepower.

Last week, BoomTown reported the departure of Yahoo’s head for the key North American market, Joanne Bradford, for a new job at social media start-up Demand Media. She starts there Monday.

It was a move that sent reverberations throughout the online ad market.

Less known, though, is that Microsoft (MSFT) has also been looking for almost a year for someone to head up its online ad sales force globally.

Yahoo (YHOO) is just starting its search to replace Bradford–with her boss, U.S. head Hilary Schneider, taking over on an interim basis.

In fact, even Yahoo CEO Carol Bartz is pitching in, giving Bradford’s staff a talking to earlier this week about the need to press on.

Both Yahoo execs said the company will be looking at both internal and external candidates.

Until then, said a Yahoo spokeswoman, in the boringest quote ever uttered: “Yahoo! has leadership bench strength and we continue to be committed in delivering wow experiences to both users and advertisers.”

Not to worry about obfuscation, as BoomTown has the scoop!

Internally, the key execs being eyeballed include 11-year Yahoo veteran Mitch Spolan, VP of North American sales, and Seth Dallaire, a former Microsoft exec whom Bradford brought to the company last fall as VP of mid-market sales, a newly-created role responsible for all mid-market sales efforts across search and display advertising.

Another former Microsoft exec, Erika Nardini, VP of brand packaging, is also mentioned a lot as a possibility and is well-liked by the sales force at Yahoo.

Still, many feel that with the exodus of such a high-profile exec as Bradford, Yahoo has to attract another big name to replace her.

But externally, the pickings are much slimmer, with only two key names popping up as top choices.

One is a former Yahoo, Jacki Kelley, a longtime online ad exec who is now North American president of Universal McCann, a unit of the Interpublic Group (IPG) agency. Besides Yahoo, Kelley has worked at Gannett’s (GCI) USA Today and Martha Stewart Living Omnimedia (MSO).

The second is Kathy Kayse, a well-regarded former AOL (AOL) ad exec, who now is in charge of digital ad sales at Discovery Communications (DISCA). Kayse also had a long career at Time Warner (TWX).

(Both are pictured here, Kelley at left and Kayse on the right.)

Microsoft is a dicier proposition, with exactly zero internal candidates considered qualified to lead the online ad sales effort, a job that would report directly into Corporate VP for Consumer & Online Darren Huston.

Microsoft has been searching for a long time now, with feelers all over the industry. A variety of names pops up, from new MySpace ad head Nada Stirratt to Bradford–also a former Microsoftie–herself.

In fact, the paucity of experienced execs to handle these complex jobs–which include the need to understand premium, network and search ad sales, as well as highly technical systems–is clear.

“Neither of these jobs are easy and, in many ways, a giant nightmare,” joked one online ad sales exec.

After its AOL division paid out an insane $850 million for social-networking site Bebo yesterday, one had to wonder if the true digital legacy of Time Warner (TWX) will be as the perpetual gravy train for legions of Web players.

It certainly seems that way, from the original AOL execs who “merged” their company with Time Warner in 2000 and cashed out at the peak right after the deal to the series of ad-networking start-up entrepreneurs who got acquired, took their payouts and skedaddled right on through to the two founders of Bebo–Michael and Xochi Birch–who didn’t even stay long enough for a latte after grabbing their chunk of the payday Time Warner was handing out in crisp bank notes for the social-networking site they founded.

And, more importantly, after one digital misstep after the next dating back to its Pathfinder days–which I have likened to watching someone fall down an endless staircase–one also has to wonder if Time Warner will ever see any of the upside of the Internet itself.

I remain dubious.

And after interviewing numerous sources close to the company yesterday after the Bebo deal was announced, I am even more certain of more trouble ahead.

Here’s why:

1. While I have always admired Bebo for its innovation and cool ideas about content (I love its “KateModern” online series, as you can see here), AOL essentially just forked over all that money for an audience of primarily teenagers in England, which is Bebo’s biggest market by far (but where Facebook has pulled to No. 1 in a year).

And while Bebo execs would argue with me about this, especially since international aspirations were touted by AOL yesterday, it has no more international traction than much more powerful leaders Facebook and MySpace. More significantly, its size in the important U.S. market, which is hoped will be helped by a marketing boost from AOL, is small and further traction remains questionable.

To be fair, AOL also touted the high engagement levels, which Bebo does have in terms of both minutes and page view per user.

2. Sources close to the company say AOL CEO Randy Falco and President Ron Grant–who are none-too-lovingly called Burns and Smithers at AOL–kept the deal a relative secret from most other execs, including those who might be majorly impacted.

It is not abnormal for acquisitions to be done in a tight group, but was apparently excessive in this case, and reminds one of the sneakiness of former Time Warner CEO Jerry Levin in the troubled AOL merger.

3. Sources said Falco had repeatedly told execs at AOL that he had to do a “big property” acquisition to move the needle, which has not been exactly moving at the unit of late, in order to show Wall Street that AOL had a social-networking strategy. “It’s like constantly scrambling eggs, by doing big new moves, you can hide the problems,” said one exec.

4. The turmoil in its online advertising unit, dubbed Platform A back in the fall, is real and profound and extraordinarily troublesome, given that it is supposed to be the engine to make the Bebo financial projections work at AOL. As I wrote earlier, Bebo needs that jump-start given its small revenues and profits.

The recent departure of three of the key executives who were supposed to be part of Platform A’s success–VP of Marketing Solutions Kathy Kayse and EVP for Global Advertising Strategy Dave Morgan in February, as well as Platform A President Curt Viebranz last week–is worrisome, even though it has been floated by AOL as a housecleaning.

But, curiously, all get good marks for competence from many and had, in fact, been recently touted as saviors by AOL. They do share one thing in common, said several sources: Run-ins with Grant, over cuts in spending and disagreement over aggressive sales projections in a recessionary economy.

In addition, all the key execs from its Tacoda acquisition are gone, along with those from its Quigo buy.

And, while its Advertising.com top exec Lynda Clarizio has taken over Platform A and is considered a strong exec and a “go-getter,” many sources told me she also reportedly had similar testy run-ins with Grant, before he recently was quoted on her promotion: “There is no one better qualified to do this than Lynda, whose track record at Advertising.com has been nothing short of stellar.”

While corporate departures and infighting are also common at many companies, especially over budgets and performance expectations, the level of rancor at AOL has been high.

5. Perhaps most importantly, it remains a mystery to me and many others I talked to yesterday that AOL has not truly attempted to take its very powerful properties like AIM and ICQ and make them more social, building applications on top of already robust ones and partnering around the Web.

“Didn’t AOL invent the social graph with Buddy Lists?” said one perplexed Silicon Valley luminary to me. Yes, indeedy, it did.

Thus, I am still trying to figure out why AOL–which was built on the pillars of community, communications and connectivity–has consistently not been able to leverage its still-valuable assets.

I suppose it is sexier to do a big, splashy deal, of course, which takes focus away–for a while at least–of the essential need to take hits, while doing the slow block-and-tackle work it will require to really build a strong ad and social network.

Buying Bebo, the third-ranked social network, for so much and trying to turbocharge it is a very lofty goal, of course, but the real problem with the acquisition is that it feels like an answer in search of a question.

While Bebo President Joanna Shields–who will enter the AOL exec team as part of the deal–and the Birches have clearly built a very interesting property, the weight of Falco’s calling it a “game-changer” on which AOL’s future rides could turn out to be much too much for Bebo to carry.

That is, especially with that heavy bag of Time Warner cash it is also shouldering.