In my experience here, that term isn't used much here anymore, which is good, as it offends some people and I think is (justifiably) considered rude.
I understand completely where it comes from, though. This is the internet, and we shorten things, hence BM. But, more to the point, it comes from the frustration of being a stepmom. You love, care for, and mother children who aren't your own, and yet the only recognition you get is to be called "stepmom" -- a title loaded with negative cultural baggage -- and your husband's ex, who in many cases will have been driving you crazy in one way or another (we used to call my husband's ex "Mother Superior"), gets the hallowed title "mom." But, then, that's life.
So I do get where it comes from, but on balance, I think we should reserve "bio mom" for mothers whose only mothering was in the biological sense. And, in this community more than many others on the internet, we mostly do that.