I have a separate Facebook page for my photos: https://www.facebook.com/JonathanHuyer and post to it maybe once a week or so. I use it because it's convenient, and the images appear in news feeds where people are already surfing (as opposed to someone needing to deliberately check out a site to see what might be new). Facebook automatically compresses the image when you upload it, so the quality is not as sharp as I would like. On the plus side, that means it's less likely to be appropriated by others. You get reasonable viewing statistics from the page so you can tell which images have more crowd appeal. I also throw the occasional photo on Instagram, although I'm not sure what the general usefulness of Instagram really is.
One advantage to social media is I can make announcements, such as whether I have a photo on display in a gallery or if I'm doing a presentation somewhere. So like Mark said, it's free advertising.