I don't think renting specific users' signatures is a good idea, but if you look through the
services and
project development boards on this forum, you'll notice that any thread offering "earnings" or a "bounty" receives tens of thousands of views compared with the threads that do not. An easy, cheap way of advertising would be to post a thread there, making sure the title advertises free BTC, and somehow offer a small amount of bitcoins in exchange for "likes" or "follows". A method that would require little effort and a fixed payout would be to do a simple raffle requiring follows+likes to enter, but a way to get maximum exposure would be to offer varying signature ad payouts after users make a certain number of posts per month with a link to your site in the sig. Hope this helps!