Im having the same problem 6 months after you guys...was it fixed for you?
When I look for a game it says there is about 20000 players online and I know for a fact that's not true and when I search for a game it will say "searching for available game" so I will back out and try again but this time when I try it will say 0/1 games found and then I can't back out or anything then it will tell me I lost connection to the COD servers. So my conclusion MW3 has died the servers are hacked to the point where the fair play bot or something won't let you find a game because the servers are over loaded with mods and fake bots.
If you downladed the DLC maps turn them off with the SELECT button on PS3 or it's counterpart on XBOX. Once you turn off DLC maps you get matched in 2-5 mins as opposed to the 5-20 mins. Hope this helps, let others know!!