Yes
4 players can play through Xbox Live, but only 2 can play on the same Xbox 360 console.
No. PS3 and Xbox has different servers.
No. Left 4 Dead Xbox Live Players can only play with other Xbox players. The same goes for PC.
4 players
No
As long as both players have Xbox Live Gold you can play 2 seperate accounts on the same console on Zombies and Multiplayer.
Yes 2 players can play NBA 2k11 on a xbox.
No, you will only see Xbox players.
You only need multiple Gold accounts for Xbox One if players wish to play at the same time. If players do not wish to play at the same time they can share one account.
2 players can play in xbox
yes normally you can play up to four player on the same xbox360 on-line
Yes with a HD and the backward compatibility update