Titanfall on Microsoft's servers or something totally unrelated, Xbox Live is currently experiencing "limited" capabilities. Users may experience issues signing in, or while attempting to do any matchmaking in Titanfall and Forza 5.
Viewers of our recently concluded Titanfall livestream will have seen these issues first hand. No word yet on when this will resolve. It's just hitting the end of the work day on the East Coast, so the Titanfall servers are about to experience a crushing load.
Update: At 10:47PM ET, Xbox Support reported the problem was resolved. It recommended users perform a full power cycle if they're still having troubles - that's holding the Xbox button down for five seconds to turn the console off, then waiting 30 seconds to turn it on again.