It’s pretty hard to have a “career” doing any single creative thing nowadays. If you really make a stir as a “science fiction writer” nowadays, you’re likely to get swept up in all kinds of network-society fringe activities, such as blogging, going to conventions, comics, gaming, TV, movies, collectibles…. The days when you could be a “science fiction writer” and work exclusively on books and magazines seem to have vanished already.
I’m pretty sure that the best way to get a toehold in writing is to start writing work that you yourself want to read. Then, see who really cares about it, and try to understand why. Wasting energy trying to ace your way through collapsing industries is a drag. You should never be surprised if your most effective, most influential writing is writing no publisher will pay for.