They'll probably IPO, but there's no way they'll be Google-sized. Google is basically a money printing machine because the dominated the most monetizable activity on the internet: searching. People are looking for things to click on when they search, so if an ad comes up with something that's similar to what they're looking for, the click on it. On Facebook, people are looking for information about other people and generally zone out the ads, which is why they get such low clickthrough rates, as was reported a while back. That puts it on the opposite end of the spectrum from Google. In between are media sites, which are generally passive, but more conducive to clicking on ads than an active site where what you're looking to do is completely different from the ads.

They will be/are profitable and will probably IPO, but I don't think it'll be huge. The numbers don't justify it.

Unless they have something crazy up their sleeves, which is a definite possibility.