So who gets to decide which buttons should be in browsers by default? Sure, Facebook, Google+, Twitter are obvious. How about Pinterest? Or X, Y and Z which might be not popular in US but popular elsewhere?
Why not let users decide? Let it be in browser settings and the users can choose which service they share to, by default.
Possibly there could be a meta tag describing which share buttons should be shown on the page--even what the particular share link should be when they're clicked.
why not just put them all there and let the user decide which ones they want shown. I mean, it doesn't matter if there's a stumbleupon thing if I never use stumbleupon. Put 'em all in. Make it easy for users to add new ones (or small ones) and hide which ones they don't want.