50,000 websites implemented Facebook social plugins in the first week

"At f8, we also launched new tools called social plugins — including the Like button, Activity Feed, and Recommendations — on more than 75 websites. Already, just one week since launch, more than 50,000 sites across the Web have implemented the new plugins. We are thrilled by the strong adoption so far as developers realize how easy social plugins are to use and how powerful they are in engaging users in a frictionless experience without requiring them to share any personal information."
Source: Facebook blog, 28th April 2010