Percent of registered users doesn’t equal the percent of active users, keep that in mind.
For a place that has very little AD’s (still non in their mobile AP), and is a free service otherwise it’s definitely a bubble value wise, especially when compared to many established websites in the past that are major players now and it’s clear where they get their money.
Facebook has potential an investors are banking on the sales possibilities. Mark my words AD’s will be a lot more common place once it goes public.