This very interesting article from a couple of months ago comes via this almost equally interesting "AMA" ("Ask Me Anything") post on Reddit, about how to build a social entertainment website.
The key point is that some businesses need to solve a chicken-and-egg problem of getting significant numbers of users before more users will join. One solution to this, which sites like Reddit or the new darling Quora have used, is to fake the initial activity, to present a higher level than would otherwise be apparent.
Of course, this doesn't mean put up false testimonials ("Great app; use daily! - Barack Obama), create fake real-time activity (extremely easy to spot), and fake your numbers (though I know plenty of startups that do, and it works).
Rather, you can engineer your appearance to give off a sense of size.
Worth a quick read.