If you’re interested in building a social network that you can actual profit from as well as enjoy, I have a simple suggestion (or 2).
Find a niche that you love. Let’s say you like Biking. Build a social site, use a service like ning.com

Build your community up, use other social sites to promote it like Facebook, MySpace, MyBlogLog, Flickr

watch it grow with active users, keep it local and highly niched. Target your biking site to Bike Riders in San Diego, CA. have the site talk about local events, local races, photos, etc.

Then once you build up a solid community of Biking enthusiasts in San Diego, I would suggest that you sell the advertising rights to ONE company.

Let them be the single sponsor for a 6 month time frame.

Getting their name and brand in front of a highly targeted audience is crucial and most companies that are trying to expand their business will be more than willing to give your new social site a sponsorship.

Make a comparison between where they are advertising now, how those dollars are spent and measured compared to how those same dollars would be spent on your site. I would imagine the money they spend on advertising now is not measured, they probably advertise their own website without being able to monitor its effectiveness.

Most companies simply throw their URL in their advertising and think they’re golden. Not true. They should be using a promo code, a unique URL, something, anything, to measure their ads effectiveness.

your social community is targeted, its made up of bike enthusiasts, average age is 32, 75% male, 65% married, 30% active racers, 70% casual riders. Average money spent a year per user is $650, etc., thats valuable information. think about it.

If you already have a niched website, take a good look at your community and see what companies, LOCALLY, would be interested in your community.

Give it a shot.