The schedule maintenance for Steam is now underway and will last three hours at most ending at 9:00 PM Pacific time.
Some Steam services are now offline during this time including the Steam Community, in-game matchmaking or lobbies, downloading games from Steam content servers, and making purchases from the Steam store. Games that do not rely on Steam matchmaking services should remain playable.
News updates and any further information will be posted in this space.
We will work to make this downtime as short as possible. Thank you for your patience!
The official article.