When people talk about marketing and social media they often forget the experience of the design makes a big difference between social site you wan to stay around in and recommend to your friends. I recently discovered Mint.com and found the UI design quite attractive and sophisticated, especially for a personal finance management service. In the spirit of this I found an interesting article on Smash Magazine today listing 10 trends in UI design that could give you some ideas for your next web project: http://www.smashingmagazine.com/2009/06/23/10-ui-design-patterns-yo.... They are not particularly social media oriented but good tips to consider for effective web design.