I’m pecking this out from the bowels of the New World Stage, where Day Two of the Twitter-centric 140 Character Conference is meandering along. But the most interesting Twitter-related news is coming from outside the conference: Data from comScore (SCOR) showing that Twitter-related search has become both measurable and meaningful.

Well, measurable, at least.

From Citigroup (C) analyst Mark Mahaney:

Twitter Becoming Meaningful–Search volume on Twitter in May was 30.1MM, (.001 percent U.S. market share), with 4.2MM searchers, and 39.4MM Result Pages, exceeding the 22.2MM searches conducted on Time Warner Cable.

Normally, we don’t even bother to dismiss companies that have less than one percent of U.S. search market. We never write about them, period. But given that Twitter boosters continually try to position the company as a real-time search engine–one that theoretically can threaten Google’s (GOOG) search monopoly, if you listen to some pitchmen, this may be the single most important metric to watch for the next few years.

It’s certainly more important than the number of Twitter followers of Oprah or even Twitter’s overall growth rate, which is going to spike up and down over the next few months as the media hop on-and-off the story (it’s back on again this week, with the Tehran protests, but we’re entering the dog days of summer, and us media folks have twitchy attention spans).

And it’s probably more important than any revenue Twitter generates over the next couple of years–the Twitter dudes have raised $55 million and have spent little of it, so they have time to create a business.

But if Twitter really is going to become an important player in search–or at least an attractive acquisition candidate for the likes of Google or Microsoft (MSFT)–it’s going to have to show a steady increase in search share.

And the nice thing about owning .001 percent of the market is that it gives you plenty of room to move up.

J.P. Morgan’s Imran Khan offered an early look at market researcher comScore’s May 2009 core search volume and market share metrics for the U.S. this afternoon (to be publicly released tomorrow) and they show what search metrics always seem to show these days: Google’s share of the domestic market growing at the expense of its rivals (click on tables below to enlarge).

According to comScore (SCOR), Google’s (GOOG) share of the domestic search market rose to 65 percent in May, up from 64.2 percent in April. Its core search volume grew 42.5 percent, far exceeding the 40.6 growth in posted in the month prior. Meanwhile, Yahoo’s (YHOO) domestic core search market share slipped a bit, falling to 20.1 percent in May from 20.4 percent in April. Its core search volume for the month grew by 29.5 percent, down from 39.3 percent.

And what of Microsoft (MSFT)? It, too, suffered a decline. Its share of the domestic core search market fell to eight percent in May from 8.2 percent in April. Core search growth was also down at 24.9 percent from 25.7 percent in April. Clearly, Microsoft is still struggling to gain purchase in the search market. But, while these data don’t yet show it, Microsoft may have found some traction. As I noted last week, Bing has been generating a fair bit of interest, and that bodes well for some improvement in its position in the search market.