So very true Shuhaijiang. A wise one!
All these sites are doing nothing but
recycling. Actually, members are the ones who get recycled!
All the popular large sites (15,000+ members) start from just a scratch, then they grow, some of them slower some faster, in some moment the entire GPF population is there activelly clicking and when they become large all the symptoms of collapsing are there. One day, their accounts get frozen and stuff

The site is gone and all the folks simply move to another one(s) and unknowingly get recycled

Smart ones, of course earn cash out of it but most of the users just pennies...
Typical examples are WolfEmails and yourPTR that I know of... I'm sure there are many more.
Cheers!