Most websites dependent on advertising do not disclose detailed information about their userbase. The demographics, number of active users etc. is arguably a key indicator of the business.
LiveJournal takes the opposite approach with being completely transparent:
On this page for example, everybody can learn that out of a total of 12M accounts only about one-sixth are “active” (not defined) and of that fraction only about one-third have updated within the past 7 days. They can also learn that LJ has a very young audience, the distribution peaking at 19-20, women outnumber men two to one, and US residents outnumber bloggers from every other country. For advertisers trying to decide if this is a good way to reach their target demographic, this is very useful peek at the audience.