Well, I think I know exactly the site you're talking about, but I won't blow your cover for you. I saw it on Reddit just a few days ago and there's a suspicious Facebook icon in the corner that wasn't there a little while ago... so if this is your site, shoot me an email (in my profile) because I found a few small HTML bugs and such. It would probably be to your benefit to share the link here though.

The SEO point is a very a good one, especially for a site that is (probably) very JavaScript-heavy. More than just your site's own tagline, you can optimize for keywords such as "social music," "music sharing," "music community," etc. I just pulled these off the top of my head -- I recommend spending some time with the Google Keyword Tool ( https://adwords.google.com/select/KeywordToolExternal ) if you haven't already.

With a social music site that's based on upvoting, you also have a tremendous resource that sites like Last.fm don't have. Beyond just allowing people to friend each other like all the social networking sites do, you could introduce a feature along the lines of "Other people who upvoted this song also liked..." as a good way to add immediate value for a new user who has just joined and voted up a few songs, but hasn't added any friends yet.