Before embarking on ANY kind of communications campaign, you should know the audience you're trying to reach. Yeah, there are a ton of great sites out there that would be neat to reach out to. But if your audience isn't there, what is the point? According to a recent post on Business Insider, 69% of adults don't really know what Twitter is. Unless your audience is part of the 31% that does understand and use the service, you are probably better of focusing your effort elsewhere.
Similarly, a recent post on TechCrunch about Friendster, shows that the social networking site we North Americans might have thought of as dead is still alive and well over there. Any kind of marketing campaigning focusing on people in that area of speakers of languages from there might be better off focusing their efforts on Friendster than on Facebook.
Not only is it a good idea to know your audience in order to know where they are looking, but it will also help give you an idea of what they are looking for. To grossly simplify things, someone trying to reach college students might be better off with a series of entertaining online videos, whereas someone trying to reach purchasers at businesses might be better off providing their audience with printable, fact-filled PDFs.