I used to - but it required a fair bit of work to start to see results from it. I basically use to trawl twitter a couple times a day looking for people who were having issues with certain tech related issues (as I ran a tech site) and where I knew I had a solution for them on my forum, I'd reply to their Tweet and send them the link to that particular post. If nothing else, it got me some clickthroughs and some Twitter follows but I did actually gain one or two members through it.