Just because many people are not using something, it doesn't mean demand for that something is high. Often, it's the other way around - if many people are not using something, it's usually due to low demand (at this price).
What price? Time, effort, attention, opportunity costs, etc.
I think it's barrier to entry. It must be very frustrating to join Twitter today, see people with thousands of followers and not having any.
Most non-famous people I know who joined Twitter recently follow 50-100 people and have a handful of followers (who know them in real life). They get bored quickly and stop using it.
Actually, that's exactly what it means. Demand = consumption.
Opportunity for growth and growth are not the same thing. I think Twitter has opportunity for growth, but that's not to say they will definitely execute on it.
What price? Time, effort, attention, opportunity costs, etc.