Honestly Flight Rising is similar to Neopets; though, it's a lot better in my opinion
The only alternatives I've ever found were just other forums. Mostly because I spend a lot of time chatting on neo, so whenever I leave neo I usually am able to because I find a forum to talk in.
Neo is so unique. It has so many addictive components to it that have never been replicated properly. Especially now where online community games are a thing of the past I think.