Some social networks can make money through advertising - for example, Facebook charges for advertisements displayed on a right banner, including monetization schemes like a cost per number of impressions or a cost per click.

Networks like Pinterest are claimed to use affiliate links to make money; for example, a user clicks on a picture of a handbag (for example) on Pinterest, which goes to the Net-A-Porter website.  The user then buys the handbag for a retail price; Pinterest would then pocket a percentage of the retail price based on the fact that it directed a sale to Net-A-Porter.

LinkedIn makes money through advertising, but they also make money through premium subscriptions (so you get better services if you pay) and recruiting services (where they compete with traditional headhunting services).