Well, content will attract people; but expect the growth to be slower if you go that way ( I did). The best way to attract people, I think is to 'do something' that will immediately benefit them in some or the other way. Plus, I've figured out that instead of a general discussion forum, target something VERY focused ( that is, instead of Apple Fanboys, start iMac Air 13" fanboys community) and that would attract a ton of people!
During initial days of CrazyEngineers, I discovered that crowds attract crowd. Create a lot of fake users ( at least 10) and start posting using their ids . Once you get 'real' people, make those accounts inactive.