Twitch still seems to hold the crown of streaming games over YouTube. It's just a better made service specifically for gaming allowing people to find smaller streams easily.
Instagram and Facebook appear to be tied in social livestreaming.
Mixxer and Smashcat are less popular then the others, but still have a large viewerbase
Dlive is still growing, and one of the major factors probably restricting it's growth is the lack of a working mobile app to watch. Dtube Viewer is very buggy and requires a little bit of patience to work properly. Dtube definitley has a lot of potential, though.
An active audience to view is obviously the most important factor of choosing what platform.