This is interesting, and I wonder just how you would measure it. Let me explain: I have adopted many profiles of my direct ancestors as well as some of their relatives and kept them, but I have also adopted many, many more just to work on them, and then removed myself as manager. (I get them sourced, connected, add Family Tree links for future users and cut them loose. This is mainly for Notables, cemetery work, merges, etc.). I've probably churned through a hundred in the last year. Would it be the profiles you adopted that you keep, or the ones that you adopted and then dropped?
I'm pretty sure there are a lot of folks that do the same as me, while there are others that are appalled by this. Either way it generates a number. Personally I'd rather not know.