Sometimes it get's a little creepy when someone you haven't seen or heard from in months, or even years, comes up to you and seemily is up-todate with what's up in your life. Has this happened to you? It has for me, thanks to Facebook and Twitter (yes, my mom follows me on both, sigh).
What I'm pointing to is that even though you post stuff and sometimes nobody comments or reply's, you're still being seen by an audience. If you track your links on Twitter with say, bit.ly, you may know that not everyone clicks through, but remember more people are looking at your post. It's even more prevelant on Facebook. Facebook, now the 5th largest country in the world (if it were a country), has more late majority adoption than any other social technology out there, less perhaps email. The thing with Facebook is that not everyone likes to post and comment, for a variety of reasons (shy, fear, voyerism, privacy issues), but they like to watch and stay up to date.
According to the folks at Forrester Research, these "spectators" can be characterized based on demographics. Take a look at the Groundswell's, Social Technographics Profile Tool and understand the tendancies of your target audience, and I recommend picking up a copy of the book as well.
And remember there may be more people out there listening than you think.