I post occasionally in 3 other places. 2 others are cycling oriented with 1 of them a local-city based cycling forum. But it is this forum I check most often. I actually find it easier to choose different topics to respond within 1 central interface in 1 site and see responses from regular as well as occasional visitors.
I do enough jumping around by visiting blogs.... So a forum feels more "stable", less fragmented from a reading perspective.
I don't use FB. Having a blog is good enough for me on sharing my favourite passions and photos. Thank God my closest friends are email oriented. They don't use FB nor Twitter either.