I just use facebook because its the first one I really joined and multiple sites I don't see a point to all they do it take up time! I think the best site would be the one that includes all of you friends because thats what matters first!
There is this site that outlines the best social bookmarking site. Have a look at www[dot]myeasyonlinepay[dot]com/top-20-social-bookmarking-sites/. Be sure to get help here.
I'm also using onlywire to bookmark useful posts. However, it keeps asking me to download the submitter when I already did. I don't understand that.
These are the top ten book marking sites listed by a popular blog site. Digg.com Propeller.com StumbleUpon Reddit.com Del.icio.us Newsvine.com Fark.com Blinklist.com Clipmarks.com Shoutwire.com I prefer Digg, stumbleupon and reditt.
it is hard pinpointing what is the best among them.. but lately I am using the new digg and it is good.. I also use reddit, mixx, stumbleupon, mister-wong, clipmarks and other.. all are very useful..