• FB ad campaign - Pretty cheap (we did something like $2/day & max of $0.50/click) and usually got more likes than clicks each day. Here's the rub though, people will just blindly like the page and not engage with it. So now we have a few hundred Likes, but I don't think people are any more likely to engage with the site. And if they aren't engaging with your FB posts, they won't see them come up in their newsfeeds anyway, so the likes (and the spend) is pointless.
• Our Twitter growth has been completely organic other than reaching out to friends to follow us, which gives a better read on people that actually care.

Generally, here's what I recommend:

• Let it grow organically. As more people find out about it and engage the product, if it's cool, they'll share it.
• Get some press. Consider doing a press release. It'll cost about $500-ish if you write it yourself. We did one and it got us featured in Fast Company, Wired, and other major sites. It led to even more press and a lot of people visiting the site.
• Reach out to blogs you think will like it. Blogs like doing interviews with founders and if it's something the blogger can get into with a lot of targeted readers, it can drive a lot of traffic for free. These people are way better to market to than rando FB users.

We've stopped the FB campaigns and are giving the site some time to grow organically now, letting people request artists themselves and working on new press. It's been a slower start than expected, but the people we have on the site are pretty engaged at this point. We just need to get an artist on that really appreciates the site and drives fans to the site to pledge.

Hope that helps!

Matt