The extension calls the function wfLoadExtensionMessages(), which was removed from MediaWiki 1.21, and so currently crashes the extension. But even after I removed the call to the function, the extension doesn't seem to be working. When I go to Special:SocialLogin, the links to the different services appear in plain text (no images or link style, is that supposed to be like that?), and when clicking them, something appears to load but nothing happens. I may have a chance to fix this myself in the following days, but maybe someone who already knows the extension will be more efficient?
Topic on Extension talk:SocialLogin
Same problem, not working in 1.22.
I think I somehow figured out how to fix the problem.
In SocialLogin.php, changing "<?" to "<?php" should work.
Because the first line is "<?", the web server treats SocialLogin.php as plain text file instead of script.