I don't think you can compare the situation with twitter or facebook. Those are actually good at what they are doing. But upwork is such a crap (in my experience as a client, for freelancers it must be horrors) that a lot of people would jump ship as soon as there would be an alternative. But sadly all alternatives like guru or freelancer.com are as bad or even more (missing words to describe those clusterfucks) as upwork. Elance was at least a bit usable. Toptal might be an alternative but their boarding process looks a bit esoteric and is aiming at another targetgroups anyway.