Where you have a follower base. ;)
For me, Linkedin is the one working the best.
Quora is more a long-term channel, so I would not recommend it for product launch announcements. If you are interested in leveraging it later check out QApop
@kate_middle I agree. Facebook is s one of the top three drivers of return on investment among all the social networks. Nevertheless, you should work with best dedicated development team to achieve success. Facebook attracts a significant user base, but you need to grab and hold attention.