Personally I use StumbleUpon, digg, reddit, pinterest, redgage and google+as well as Facebook. Note that most of these only give you nofollow links, redgage and google+ being the exception. However, I think that is fine, I get some direct traffic from these sites and there is an argument that one should have nofollow links to make one's link profile "look natural", although I have always wondered about that, since the links are basically telling Google bots not to follow them to see what's at the other end, are they even indexed? I don't think I ever see links from Facebook in my Google Webasters tools page.
I had great hopes for Pinterest because I heard it gave you dofollow links, and if you have good, original photographs on your sites, they will be re-pinned by other users, hence spreading the link. I've recently found that Pinterest has now made the links nofollow, although I have heard that if you put a link in the description of your image, it will be dofollow.
Another site that is very useful for some people, is LInkedIn. It's not something that that I want to use, because I mostly associate it with businesses etc. but if you're sites are on the right topics it could be very useful. I think the links are also dofollow.
A site that I have just started using, that gives you dofollow links, is tumblr. Not strictly a social bookmarking site, it's a microblogging platform, but it gives people the chance to reblog your posts, which are usually very short, often a photo or a quote. This again raises the possibility of a link, which is embedded in your blog, spreading to other users and creating multiple links for you. The only big problem I've discovered with tumblr, is that it is completely addictive! I find myself spending a lot of time on that site.