Most Helpful Guy
Because he misses the spiritual connection he shared with you. All relationships are spiritual, whether people admit it or not- that is why he feels lonely and empty inside- so instead of weathering the pain of being away from someone he used to care about, he needs to immediately find someone else to fill that gap.
It is called, "The rebound"
Most Helpful Girl
Because you don't want to be with him. I think that he really wants to be in a relationship, but since you no longer want that, he is going to go elsewhere to get it. I think that the dating site is mainly a back-up to be honest. I mean, what was you expecting for him to do? Did you want to crawl back to you. Just asking. Do you want him back, if not, him being on a dating site shouldn't really matter.