I think it's enabled by default, but I thankfully have deactivated it as well.. but 'coincidentally' the button to activate it is right next to the send/smiley button, so sometimes you accidentally activate it without noticing.... And yeah, I totally agree with you that it's sad that there's no alternative.. I wish I could get rid of it, but there's no way I could keep in touch with my friends and family spread over the world. I know a couple of people who tried to get rid of it, but they said in the end they just missed out on so much so it wasn't worth it
Lemme tell you, messenger HATES it if you tell them you don't want notifications. I told them no because I was pissed I even was installing it, and every time I used it afterwards it asked twice if it could turn on notifications. It was very insistent.