Relating to this site specifically - comment on what they are writing. As a new writer, I was (am) very appreciative of anyone who takes the time to actually comment. And yes, some of the onus is on me too, which is why I always try and respond. Sure it's (kind of) nice to see that people have read something, but comments are significantly more encouraging than just a number. Beyond that, there's actually discussion and conversation although that seems to happen in a more unstructured way.
As @Mikiesboy says, get involved both as a reader and as a writer.