I agree with sanj. It has to be opt-in. I realize that's probably a bummer, but Twitshirt recently got into some hot water for pretty much the same dynamic, it was opt-out, and people were like, "how am I even supposed to know about this site?!?" There was a big backlash, they took the site down until it was only opt-in.

Plus, the nature of your site, unlike say favrd (which is not opt-out, but you'd only appear there if people loved your tweet, and would never appear there if they didn't) is something people naturally feel self-conscious about, appearance and attractiveness.

I do think the functionality and design is really nice though, you did a great job.