I've heard Twitter is the best and has the largest writing community. It is also a good place to find beta readers and the like. I, however, hate Twitter and use Instagram. It's so much cleaner. I don't know that it really promotes my novels, though I have followers who are readers, but it is a great way to connect to other authors and comic artists (who often are/become your readers on Tapas as well). And I agree on keeping a separate account for your author persona.