Littlebigplanet 2 is an extremely active community and if you play that you probably won't need to rely on anything else for a while. Especially if you made friends since that game is essentially limitless. It is no longer just a platformer and improves on everything LBP1 did and takes it from a platformer to a platform for games on its own. Millions of levels and millions of players join to make this game what it is. A defining reason to own a PS3.
Red Dead Redemption is also an amazing game that I regret putting off. It gives you a massive western influenced land to explore and the multiplayer free roam is one of the most unique games I have ever experienced this generation and something that PC gamers have envyed since 2010.
Killzone 2, also a great game but you may end up switching to EU/PAL servers quite often as the game isin't as popular as it once was in NSTC. I usually play EU when I do play Killzone 2. Great game, and warzone is another unique thing that you won't find in most FPS. Combines serveral different aspects and goals combining TDM, CTF, Defending versus planting bombs at strategic areas, strongholds, etc all in one match. So you get several flavours and constant change ups.
I haven't played any of the Uncharted games online but hear good from them. One of the more popular games on Playstation3 so I will eventually get to that. I did enjoy the couch co-op mode in U3 that I did with a friend. Its a fun TPS experience.
One of my top favourites though is Starhawk, gives you a TPS with an emphasis on vehicles with large maps to explore. The basic 3 modes since launch were team deathmatch, zones, and capture the flag. Now these maps can be extremely massive making the use of vehicles essential in this game. You may have to utilize a sidewinder, a razorback, a hawk, a grizzly, or a tank to achieve certain tasks depending on what the situation calls for. If you are a fan of typical deathmatch, capture the flag, and love TPSs, flight combat games, and large areas to explore and conduct combat over, then starhawk is your game.
My favorite multiplayer is Might & Magic: Clash of Heroes...
Nothing will benefit human health and increase chances for survival of life on Earth as much as the evolution to a vegetarian diet. -Albert Einstein