I can't remember specifics, but I do remember reading either some corporate lawyer opinions or maybe a low court decision that this was valid (or maybe they had to notify, in some way, that the TOS had changed by email or with a flash message on login).

I have certainly read plenty of lawyer opinions that it's blatantly invalid to have "this contract can change at any time with no notice" clauses in a contract but I'm not sure what precedent is out there.