Mastodon started gaining popularity towards the end of 2016. At that time it was based around a single instance called mastodon.social.
It was initially viewed skeptically by the users of GNU Social instances, as being a cult based around the personality of its young developer, Eugen Rochko.
This system was promoted as being like Twitter, but with a zero tolerance policy towards far right or alt-right agitators. The "lack of nazis" was enough of a motivator to get some users to move over from Twitter.
Mastodon introduced a new concept to the fediverse - the content warning. Content warnings could also be considered as ultra brief content summaries, allowing the user to rapidly skip over posts which were probably not of interest.
What content warnings (or one line summaries) allowed users to do was to follow numbers of other users above the Dunbar limit and without necessarily having a high degree of affinity with them, while also avoiding the cognitive overload which would otherwise result. In effect, content warnings were an alternative to the algorithmic timeline, producing the same compressing effect without the down sides of censorship and shadowbans.
People on Twitter had been requesting user interface changes to improve usability and getting nowhere for years, but were happily surprised that the Mastodon developer was much more responsive to suggestions.
Mastodon initially set its character limit to 500 - much higher than Twitter's 140 at the time, but typically lower than many GNU Social instances which defaulted to 2000.
A variety of factors contributed to the early popularity of Mastodon, and one of those factors was its similarity to an already familiar Twitter user interface, called Tweetdeck.
From early on Mastodon seems to have attracted many artists, and as a consequence it tended to have better quality artwork for its logos than other fediverse projects - and Free Software projects in general.
With the rise of Mastodon in 2017 the gender composition of the fediverse noticeably changed from being mostly masculine (maybe 80% at a guess) towards being much more even. Mastodon had broad appeal and brought in new demographics which were not confined to people interested in technology.
The First Million
At the end of 2017 Mastodon had a million user accounts. However, taking into account that many of those would be dormant, or bots, a more realistic estimate of active users would between 100 and 200 thousand.
New Design Features
Mastodon also introduced new design features which were intended to reduce the potential for harassment. On Mastodon systems you could search on hashtags, but not do arbitrary searches of an instance for arbitrary text.
This was an attempt to mitigate lowbrow adversaries who would otherwise search through the system looking for keywords (like maybe "feminist") and then dogpile those users with insulting posts.
Lowbrow dogpiling attacks of this kind had been common on Twitter for a number of years, so anything which frustrated the most dim-witted adversaries was still useful.
Mastodon was the first web application in production to adopt ActivityPub for server-to-server communication, and it was available at the beginning of 2018.
Mastodon adopted the ActivityPub protocol at the beginning of 2018. It only used the server-to-server part of the protocol, and not the server-to-client. The implementation of ActivityPub within Mastodon then became primary reference for other instance software, such as Pleroma. The earlier OStatus protocol continued to be supported.