I have a vague theory that the daily number of active users is dependent upon what the rewards can support, so while there is huge volatility in the numbers of new users each day, there is also huge turnover of users, and it all balances out in the wash unless the steem price moves significantly.

Steemit and busy and esteem have been around for a while (relatively!) so have some first mover advantages. The newer platforms for video and streaming and memes are still very new but my guess is by the end of the year they'll be huge.