One other thing to consider: Google is sitting on something like $56B (and growing) in cash and short term investments. They could set $4B on fire and it really wouldn't alter their balance sheet significantly. Given its big push into "social" over the past couple of years, I could make the case that it's worth spending some of its cash to try and acquire a bunch of (admittedly fickle) users.
Or maybe the return on the $4bn investment isn't worth the brain and manpower necessary to wrestle the app into some kind of monetization scheme?