Most of those examples were not faking anything. Rentoid actually supplied a serivce, though it wasnt "user-generated". Reddit supplied links their potential users would be interested in.
This is a mixture of demonstrating how the service works, and supplying actual content to potential users.
By way of contrast, what the dating site admins should have actually done, is ask all their single friends to sign up (and any other single people they could get their hands on directly).
It seems a perfectly good and moral business strategy to generate your own content which is representative and useful. I think "faking it" is a bit more questionable.