Do you really think I haven't considered all these points? Do you really think I haven't considered alternatives? I have, and I have, and my conclusion is that I (a) want to stay in touch with these people, and (b) have no effective alternative.
These are people I care about, who care about me, and who are, today, using FB almost to the exclusion of anything else because they find it convenient and have given up nearly everything else. Despite many attempts they are unwilling or unable to use email as effectively as they use FB, and proliferating platforms would do them no favours at all.
You, and several others in this discussion, are using what you believe to be ironclad reasoning to replace any sense of understanding, sympathy, or empathy.