In past few years I have made number of communities/groups etc on various social networking sites such as Orkut or Facebook just for the heck of it. Normally, only people you know would join it and that would be it.
Recently, I made a group on Facebook and just because I had nothing better to do that night I almost ended up sending around 50 invites to random people to join it. Half of them did, and a chain reaction started. 25 turned into 30 and 30 into 50, its not even been 10 days since the birth of the group and it already have 270 members as I last checked today.
So yes, whenever you need it, I am suggesting you should go for the advertisement, it pays off. Specially, if your product/service is of more important nature.