I left Freelancer.com because, over time, it became less lucrative for me. I talked about this in detail at the end of a blog post: How to Close Freelancer.com Account.
I’m hanging on to my accounts at Guru.com and Fiverr.com because, as Igor said in his excellent answer, it’s good to show up from time to time and meet new people.
Ultimately, freelancers have to maximize their time. For me, that means focusing on my direct clients and the currently most lucrative platform for me, which is Upwork.