Scrabble, for example, lends itself perfectly to asynchronous, light, casual play (cf Scrabulous, Words with Friends). But it's legitimately fun at the same time. You're rewarded for imaginative approaches to solving problems. You can play with no skill or lots, but either way, you'll always find ways to get better.
Most important: You're genuinely spending time with your friends. Sharing an experience that creates a memory.
This is a big deal. This is missing from almost every other pseudo-social game.
True story: I was job hunting, someone gave me a lead at Slide. Awesome guy, gave me good advice. But as soon as I found out I had to be conversant on the subject of a "game" called "SuperPoke Pets!" I vomited a little in my mouth and moved on to other options.
Social game companies: Stop making horseshit, start building something fun. It's possible. It's just a little bit harder.