Adblock-like plugin for hiding social sites' widgets, buttons, etc

Ghostery does that; works with all major browsers.

Also, the "Antisocial" subscription for AdblockPlus.

The Fanboy Annoyance Block List for AdBlock Plus in Opera, Firefox and Chrome:

Blocks Social Media Content, In page Pop-Ups and other annoyances; thereby substantially decreasing web page loading times and uncluttering them.

For Firefox, I use the Greasemonkey add-on and the Remove Facebook Like buttons (mod1) script.