I'll be the contrarian here: while I love this site, and highly recommend it, I have to say that I think LinkedIn is just a tad more useful as a social network. I view this site as being very useful for a specific set of information (all things stage management), but it's very specificity (while good for info and tips) limits it in terms of connecting with other professionals outside this subset of the population. Just the other night I was at a local theatre's meeting planning their next season, met someone who also works in the film industry (which I'm trying to break into), and we exchanged contact info. I then looked him up on LinkedIn, connected, and now have a dedicated means of sharing my experience with him in all my various professional lives (outside of just stage management).
Bottom line: SMNetwork is the place to go for pure SM matters; for everything else, there's LinkedIn.