44% of Twitter users have never sent a tweet.
That statistic comes from a 2014 study, but given Twitter’s lack of growth in the past few years,
It is unlikely to have changed all that much. Added to that, a study done only this year suggests that around 48 million (between 9-15%) of the current user base are bots. Of course, a fair percentage of those are not spam bots, but useful, humourous or otherwise ‘beneficial’ automated scripts. But, from an advertising perspective, they are still 48 million fake accounts that should be excluded from the overall figures of followers/potential reach etc.
This is a really useful article which lays out just where Twitter is – in effect, many people disliked the need to start a network from scratch (unlike Facebook) and felt they were shouting into the void. There’s a lot more and I highly suggest that you read it – but the other main point is that Twitter may eventually to relegated to a niche platform, which also limits it’s advertising potential in the long term.