I have bought many games with an multiplayer (mp) component and i really do mean to play them, but unfortunately no matter how good they may be, i never do.   Whether it be red faction guerilla, transformers war for cybertron, red dead, halo reach,  MNC, cod, SSFIV, blacklight or even BFBC2; I find myself  eventually getting bored with the MP no matter how good it is. 
Now why is this?  Could it be the game sucks?  Maybe. Sometimes MP is added just fill out a feature set. Thats not always the case. Often the MP component is quite good.  So what is it?  Low player base? Possibly, but not always.  Mostly i find i want to play online with my "friends" and if they aren't playing it, then i'm not playing it.  Now this isn't always the case either.   So what is it?
I guess the big thing is that when you play MP, whatever you're doing has to be FUN to you.  If it isn't then it will get  old fast.  
I like to play a team based game as a team.  With everyone communicating and coordinating.  Which i guess is why i play with friends because thats how we play.  
When you play with strangers there doesn't seem to be the same camaraderie or respect that friends have for each other.  Too many people are jerks, bad sports or down right rude, and that get old too.
 
So i guess that sometimes multiplayer  fails because of the game.  Sometimes multiplayer fails because of the players.  But if you really enjoy playing the game then the faults don't matter.