If you're looking to get a little Left 4 Dead 2 in tomorrow night, and you don't have any friends to play with, you may be left out alone with the zombie horde. Why? Steam is performing some maintenance on its services which includes Steam matchmaking services.
While you should still be able to play single player titles, any title utilizing Steam's native multiplayer support won't be available for online play. Other services affected are the community, store, and in game lobbies. The planned outage should last 3 hours from 6PM Pacific to 9PM Pacific.