I'm finding that StumbleUpon has been the most effective social button on the site. I've been consistently sharing my blog entires on StumbleUpon and other content sharing sites like Digg, Reddit, Twitter, Linkedin, +1, Facebook, and maybe a couple other ones. It's mostly a waste of time except for StumbleUpon.
Senh: I find the two buttons redundant, but most sites still use both.
Senh: This article does mention some stats on referral traffic from Facebook for sites using their social plugins, even though most of them are in the "we don't have enough info yet" variety.