It's not that a service can't stick around post-acquisition, it's that in any company, you can have sweeping changes on the turn of a dime. One large senior management change and entire departments can get shut down or reorganized within a week.

I've seen a $1B public company start winding down a massive revenue-making division within the course of a conference call. Parse is a great service; I certainly hope Facebook doesn't do that and don't imagine they will any time soon, but nothing is impossible.

EDIT: And though you certainly know Facebook's internals better than we do, as an outsider, the risk of an acquired department shutting down is a lot higher if there's a potential to not part of the core business strategy.