If your sending tweets for example into twitter on all new threads posted on your forum, why would your friends now need to visit your community as much like before to see what's new there? And simply because you decided to start sending forum content into it. There's also a chance of course, that they might start talking about threads sent into Twitter or Facebook there itself, rather than visiting your forum board and posting a reply back on it. Just want to get your views on this one generally as a forum owner yourself?
Do you ever stop for a second and wonder if the way you use social networking sites (to try and promote your forum), is actually doing it more harm than good.