This past Saturday I was playing Magic: The Gathering (possibly the best game ever made). In one game I was getting attacked by many squirrels (bear-sized squirrels) and the only card I had that could get me back into the game cost more than I could pay. I used Jace, the Mind Sculptor to dig for a way out. I found Time Walk, buying me an extra turn (this was a vintage rotisserie draft). On that next turn I used Jace again, this time finding Channel and Frost Titan. I used Channel (taking me down to 1 life) to cast Emrakul, the Aeons Torn. In addition to a huge guy, that bought me another turn. Emrakul's attack took away half of my opponent's board. Just to be sure, I cast the Frost Titan to lock down his Gaea's Cradle. He didn't have enough guys to get through, but he did have a Banefire, which he cast for the last 1 damage to kill me.
So what does this have to do with Social Games? If you play enough Magic, you know I just told you a cool and somewhat hilarious story. The story is so interesting to me that I was willing to tell it to you even if you don't play Magic, putting in almost enough information for you to be able to understand it. Great games give players great stories to tell. I believe the lack of great stories is a problem for social games. When's the last time someone told you a thrilling tale of how they waited 15 minutes to harvest tomatoes?
To get a great story you need interesting interactions between players, or between the game and the players. Role-playing games do this best. (I have been regaled countless times with a friend's previous night's Dungeons & Dragons adventure.) Paper RPGs combine complex interactions between people with a fantasy world that has epic storytelling possibilities. Players can do anything and the results are as awesome as their combined imaginations. Video game RPGs are more limited, but the graphics and well-composed worlds give players a lot of fuel to imagine the lives of their characters. Good writing will deliver great stories, and close-fought battles give the player emotional moments they won't soon forget.
All of this is absent from social games. There are no RPGs (Mafia Wars and its myriad of clones are NOT RPGs. Don't believe anyone who tells you that*). There aren't any complex player interactions. There are no close encounters with surprising turnarounds. Social games could really use some of these things. These are the kinds of things that can dramatically increase player retention in a game. Also, if players are telling their friends what happened over lunch, you'll get some real-life viral spreading.
* Technically they are games in which you play the role of a mafia boss or similar. But there are very few games where you don't play the role of something. I guess you don't do it in party games (Pictionary, Time's Up) and word games (Boggle, Scrabble).