The 217 million active daily users? It is "only" $~80 million in the US, I'd love to know how a quarter of the US population using it daily represents an insignificant user base.
Near as I have ever seen, most English speakers actually do mean something more like that when they say "all": maybe it would be good to mentally model it as "it feels as if it would be hard to choose at random and not find this statement to be true".
We're not even over half in this case. If this person actually believes most people are already using twitter, it's clearly a problem when it comes to informing a judgement of stagnant growth.
Sort of? The thing to realize is that your average speaker leans heavily into the "no true scotsman" fallacy in order to make sense of the world quickly, and so they heavily bias that random distribution from which they are selecting candidates. While I wouldn't personally say "all" in this exact case, it certainly does feel to me as if there isn't anyone new for Twitter to really target, as you have to first discount most of the old people, "all" ;P of the young people, and a depressing number of people who really don't have the resources--time or money--to buy into technological society. Do you seriously believe there is some useful untapped market of people using other social networks--or are even in the target market for a social network but who somehow haven't started using one yet--who aren't using Twitter? I hate Twitter--I'd dare say it is by far my least favorite major social network by far--and even I use the damned thing :(.
