In Google+’s case, I do believe them that they have 500 million accounts, but that’s about half as many as Facebook. Does anyone believe for a second that Google+ has half of any other important metric as Facebook?
I think Google sees Google+ as an extension of the Google brand experience itself, which is probably why it’s both confusing and unused. They even call removing a Google+ profile “downgrading” (via Jim Dalrymple). Maybe Google’s long-term strategy for this is to bundle everything into one big Google experience (though that seems like a dangerous idea).