A statement from Sega offers apologies from Rebellion for issues with the recently released Aliens vs Predator demo. Here's the part of the statement relating to the PC version:

Unprecedented demand for Aliens vs. Predator following the release of the cross-platform demo yesterday morning brought to light matchmaking issues on the PC platform resulting in players having to wait to join games, or even not find games at all.

We’re pleased to say later in the day the issue was corrected and a small patch to the PC executable next time you launch it from Steam will have you finding and playing games using Quick Match as it was intended.