They've got a bit of an uphill battle after waiting this late to make an entry. I'd never rule Google out, but after the first wave of, mainly, devotees, they'll need to come up with something really interesting to continue making headway.
After Google Wave, and Google Buzz we'll just have to wait and see. If there isn't a breaking point for facebook I just don't see this happening. We all know the real and original purpose of FB, it may be evolving into something more than what it was intended, but without losing the bodies it won't lose any ground.