Here're what I did:
I attached myself to one of the already well known forum (XDA-Developers.com) and post regularly with a link back to my service showcasing how my service was better than what people were doing on the forum.
Since I am the user of my own service, I also keep feeding new content regularly to fake that there was activity.
If you are the user of your own service, it is easier to ask yourself "where would I be socializing", and "what kind of content I would like to see".
Ask these questions and fake it until you make it. :)