Um, no. I don't buy this line of reasoning.
People do not agree to participate in experiments when they sign up for social media. Even if, by some legal handwavery, this can be construed through an acceptance of the terms of use, it's still unethical.
Facebook is a big company. If they do not hold themselves to high ethical standards then perhaps they need to be held to those standards by the society upon which they are constituted.
So to answer OP: yes, Facebook can ask users if they want to participate in research. They should seek informed consent from all participants and ideally they would have all research proposals reviewed by an independent ethics group.
Um, no. I don't buy this line of reasoning. People do not agree to participate in experiments when they sign up for social media. Even if, by some legal handwavery, this can be construed through an acceptance of the terms of use, it's still unethical. Facebook is a big company. If they do not hold themselves to high ethical standards then perhaps they need to be held to those standards by the society upon which they are constituted. So to answer OP: yes, Facebook can ask users if they want to participate in research. They should seek informed consent from all participants and ideally they would have all research proposals reviewed by an independent ethics group.