Bebo and Six Apart are nice additions, but it’s MySpace that will bring quite a bit more heft to this effort–an addressable market of some 70 million active monthly users, at last report.

Who’ll next join OpenSocial? Will it be Facebook? CEO Mark Zuckerberg’s likely puzzling that question out at this very moment as he waits for an invitation to join–which apparently hasn’t yet been extended. “Despite reports, Facebook has still not been briefed on OpenSocial,” the company said in a statement. “When we have had a chance to understand the technology, Facebook will evaluate participation relative to the benefits to its 50 million users and 100,000 platform developers.”