Reddit to be honest and it's still my #1 source of traffic. Then Twitter (I build in public in there and always share my progress) and then ProductHunt (just launched last week).
Instagram for us, but we're based in South East Asia where Twitter is not as popular. We're actually starting to see good traffic from reddit as well (only recently past year) so it can be a good platform for founders to check out.
Movydick: And The Oscar Goes To...