I have long known the value of Flickr, but never really understood it as it applies to my own work. I stopped uploading to Flickr when I hit the free 200 picture limit. I didn't see much point, especially since it did not seem like anyone was seeing what I was sharing anyway. Turns out, I didn't understand how to use Flickr well.
In the past week, I have really been playing with it more. I have started joining different groups and sharing my shots there - "sunsets" "Alaska" "Hawaii" etc. And it really makes a difference in your traffic. I still don't have a lot coming, but it is picking up.
I have recently signed up for the Digital Photography School Forums. I've long been a lurker, but I decided I should attempt to get involved. There is just so much information on that forum, that it really is an overload. But I learn something new each time I take a look at it, so that's worth something!
Deviantart is a good tool for networking. You can "Deviantwatch" other users and be notified when they upload something. Whenever I upload any photo, I usually get a range from 1 - 4 comments or favorites for each shot, just from it showing up on the "newest" page.
Facebook Fan pages. I created one of these, and it has been intriguing to have people fanning my page who I don't know in the least! They are friends of friends. It hasn't generated a lot of interest to my site or blog, but it is creating several comments.
Of course, there is blogging as well! Blogging has been a huge blessing to me in this way. I've met a lot of great photographers and learned a lot from each of them as well!
I haven't done much with Twitter in following other photographers really. I could do #togfollow on Follow Fridays, but I haven't yet. However, that might be a good resource for you to look into.
In using these tools, I think that it is giving me valuable feedback in seeing what does and does not work, what is successful in business and in eye catching images. They are also great for networking with other photographers! I'd encourage you to check any or all of them out! Any other tools you would recommend?