The referenced line in widgets.php registers widgets for use on your site.
The Facebook plugin registers widgets your site may choose to include in one or more sidebars. Version 1.1.9 includes a name change: Subscribe Button is now known as the Follow Button to match a change on Facebook. The Subscribe Button is no longer registered; Follow Button widget and class are registered instead. The Subscribe Button widget class and file no longer exist, replaced by the equivalent Follow Button widget and class.
Which web host are you using? Are you registering any widgets previously defined by the Facebook plugin in your own theme or plugins? Have you tweaked PHP caching or anything else that may have saved an old copy of plugin files?
