Update (6:48PM MST): Social media notifications appear to be working for most folks now.
Windows Phone users are reporting that Twitter social media notifications – such as those found on your Me tile, under Notifications – have been spotty or (in most cases) stopped flowing to Windows Phone since Thursday (UTC-7). A cursory dissection of the communication between the phone and Windows Live APIs, shows that API requests for notification activity succeed but no actual results are returned. (i.e., the phone is working just fine.) Maybe it's time to look at grabbing a real Twitter app? (We've reviewed quite a few.)
Microsoft was asleep and therefore unavailable for immediate comment. We'll keep an eye on this and let you know when things are moving again.
Updated: as of 8pm EST, notifications seem to be working again.