Test, test, test. There are lots of programs out there that let you do testing on landing pages. Watch your analytics. Get a good analytics package. Google Analytics is good but not really that great for deep study of traffic. Look into heatmap software as well. This will show you were people are clicking. I have found on some websites that we had areas that people thought were links but were not. Watch bounce rates on exit pages. Keep doing testing to try to improve that rate.

The biggest thing that I can say is make sure at least one person spends a lot of time on your website. People will stay at an active website. The biggest mistake that new social network websites make is leaving it alone. They think they can spend a bunch of money making a cool website and people will just flock to the website and hang out. Even if you spend a bunch of money on advertising people will not stay if nothing is going on. You have to build a core group of members and get them talking. Also get involved on other websites. If you have some information on your site that answers a question on another site be part of the conversation and politely leave a link to your site where the question is answered.