i don't know how contrived the examples are for facebook, as i am not on it, but replace it with wechat in china and you have reality. many people really have no other way to keep in touch with some people.

apart from that, i have relatives in another country, and neither of us have our phones set up so we can make international calls (because that costs extra money), so while we could get in touch, we wouldn't unless it was something urgent. instead either of us would just wonder why the other is staying silent for a few days. and some relatives just refuse to use any other way to communicate besides the one of their choice. it's not facebook fortunately, but still, the example is not really that contrived.

i also have many friends that i can only reach through one method. if i loose that method, they are gone unless i am lucky and i can reach them through intermediaries.

ADDED:

temporarly blocks may not be that serious, but permanent blocks exist too. we have seen many of those stories even here on HN.

the problem is really that i fear many do not think that this could happen to them, so when it happens, they are caught unprepared and unaware.

i certainly lost contact to some people because we didn't consider this a possibility.