When I’m asked which writing sites I prefer, I usually send people towards Absolute Write or Zoetrope, which are free, or Write Words, which charges for membership. These places combine writers’ message boards with places to post work, and to give and receive criticism, and are populated by real, professional writers, editors and agents who know about writing and publishing, and who are keen to share their knowledge with others. The criticism can sometimes be strong but it will, generally, be constructive and well-informed and all writers, of all levels, will learn a lot about publishing on the way.
I am wary about sites which are full of writers who are as yet unpublished, or have only self- or vanity-published, as they won’t necessarily have the experience or expertise to help other writers discover where they are going wrong. Many, for example, suggest vanity publishing as a way to go. However, some do offer a unique and nurturing community for the writer to grow in, and so can be worth considering.
It really depends what you’re after. If you want to participate in a fun writers’ site but aren’t particularly interested in commercial publication, then try them all. If you want to get properly published, and perhaps want to make a living as a writer, then you’d be wise to join one of my favourite three. Just be aware of each site’s limitations, consider what you want to get out of your online time, and do your research, and you should find somewhere that suits.