We don't know how many people sign up because someone said it was cool or invited them and then forget that they have a Facebook account. It serves Facebook to

We don't know how many people sign up because someone said it was cool or invited them and then forget that they have a Facebook account. It serves Facebook to publicize a giant number to make them appear unbeatable, but the numbers that count aren't about number of users - they are about usage. I think it's a reasonable analogy.