While it's widely accepted that advertising on social networks can be a mixed bag in terms of results; for certain types of campaigns, it may be worth giving a shot.
If you're interested in dipping your toes in social network advertising, here's a list of the ways you can reach users on social networks.
Self-Serve Direct Advertising
Both Facebook and MySpace offer self-serve advertising platforms that enable you to set up and manage AdWords-like campaigns on their social networks.
Other popular social networks sell advertising directly but don't offer self-serve platforms.
With most of the popular social networks offering application platforms of some kind, one way to reach users on social networks is to advertise with popular application developers.
Application Ad Networks
A number of ad networks have sprung up to help smaller application developers monetize their applications.
Here a several of the more prominent ad networks:
A Few Words of Advice
Although social network advertising has its limitations, if you are aware of them and remember to apply the basics of campaign development and management (measurement, testing, etc.), you might find that social network advertising can be a small but still useful part of your marketing mix.