There must be a reason for them turning to something else than

You may be new to modern software development. Switching proverbial horses is massively common, usually for no benefit (or to lock people in). It’s everywhere, and especially in the corps who want that lock-in (ohai apple).

For a 2 week overlap, people on gtalk, Facebook and a regular jabber server could chat with one another as easy as addressing an email message. Then both Facebook and Google switched to their own homebrew replacement and made up some compelling-sounding, feature-laden but implausible reason. Gtalk has never sucked less than those two weeks with a working discreet client app and interoperability, though. It was actually adequate.