As I'd posted this poll a week or so back, general direct-dialed telephony, a/k/a PSTN (public switched telephone networks) seem to be in trouble. I think we could be within five years of their total collapse. And no, not just land lines (already about 25% of their peak in much of the US), but _all_ direct-dialed phones: mobile and VOIP included.
The problem, as I'd written before, is paradoxically the _low_ cost of calls. This is inducing tremendous volumes of junk calls, with one ...
I'm more than happy to be proved wrong. But past systems seem to have first shown issues at about 100k users, and hit substantial problems as _actives_ climbed into the 1m - 10m range.
The fact that Facebook _has_ scaled to _billions_ is actually quite an accomplishment. The problems have finally appeared, but they were kept in check for quite some time.
For open and federated systems, there seem to be a number of failure paths, though I'll save those for another discussion.
I know the feeling ;)
Generalistic and moderated instance.