Like all forms of advertising, social network advertising can be a hit or miss. It seems to be even more so for sites such as FaceBook and Twitter where the majority of people are. For my old forum, I tried Twitter and FaceBook. Both actually got me better results than I had anticipated. My current forum, which is more based on my real life community, has actually gotten a lot more of my community on it thanks to FB specifically.
Despite how many people are using the internet these days and know of sites like Twitter and FaceBook, they will have no clue what a forum is. A few of my members said they weren't even aware there was this type of site around.
Has advertising on social networks helped you at all?