Like a few others here, I'm skeptical of the benefit of using social media to promote your stock photography. While there may
be some benefit, one has to also consider the opportunity cost. How much time spent on social media could be spent on producing more images?
In any case, there is one aspect of social media that I think is invaluable to the stock photographer - and that is blogging! But not necessarily for any exposure it might get you, but rather just the act of trying to illustrate your own blogs with your images gives you a sense of what might be useful to have as content for sale. So get out there are blog, as a learning exercise. Also, read other's blogs to get ideas on what kinds of things might be useful to bloggers who buy images online.
This shot is one I came up with trying to think of a way to illustrate a sun burn, and the need for sun screen. Some cells were harmed in the making of this photo, but rest assured I waited until someone accidentally got a sunburn!
Just doing this blog helped me find a subject with only 114 results in a search!
I suppose if I'm going to say one should blog, I should pass on my actual blog as well: High Fidelity Compacts