I think that there are three types of these 'social' games available on facebook.
The first category is the kind of game that ranks competition above all else. Something like Facebook's version of Tetrisfriends (which in my opinion declined horribly after becoming part of the FB) in where the stats and rankings of any of your friends it displayed. You can compete, and even have heard to head matches, and once daily send minor boosts to each other. You also play competitively against similarly ranked opponents, though you receive no bonuses to gameplay based on your friends list.
The second category is a game like Crazy Planets, where (at least in the beta version I tried) never actually encounter enemy teams in multiplayer, yet your team consists of you and three of your friends, who are just as powerful in they're games, yet only as skilled as you are, since you play as them. This is good for enjoying a game with friends who have similar tastes in games.
The last category is the one which I think causes the most rage on the issue. Games like MafiaWars and MightyPirates, where if you want to be able to participate in multiplayer without getting mutilated you need to either spend real currency on the most powerful weapons, or have a large list of friends to back you up. I know this to be especially true in mighty pirates, where you can only have one member of your ship perform one action per turn, which limits your survivability fircely. If your alone, you can shoot, aim, or make minor repairs. and that's you entire turn, where the opponets with their crew of ten can shoot five times, aim four to improve accuracy to about 100%, and repair your single shots worth of damage.
These types of games lead to people inviting random users as friends on forums to grow more powerful, which I think in part leads to security issues as now the information you only provide to your friends, or friends of friends, is available to strangers with possible malicious intents.
Edit: sorry about the triple post, I is a forum newb :/