When Paypal was still part of X.com (1999 or 2000?) they paid new users to sign up ($10 added to your new account).
I'd be interested to hear more stories like this about seeding sites successfully without appearing fake. I've seen a lot of startups seed their sites in ways that just look too obvious.
The terms "two sided market seeding" seem to yield good search results if you want to explore further.
That phrase is new to me. ikeboy, thanks for the tip.