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.
@FiXato The blog is ... on my to-do list.
(The to-do list is ... not on my blog.)
I know the feeling ;)
Generalistic and moderated instance.