eleonorang It all depends on personal preference, imho. If we talk about marketing for the sake of sales and bringing in new clients then I'd say that social media in general doesn't bring immediate results. However any social media that you choose can be very instrumental for networking and positioning yourself as an expert in a certain field.
Either way, to obtain meaningful and measurable results you'll have to be working on your social media full time (creating content, participating in discussions, following topics, coming up with new initiatives and promotion tactics). It's a ton of fun, and every social network has it's own pros and cons, but at the end of the day many of us simply cannot afford to spend all their day on social media.