Ascended Fanon: Twitter likes to officially implement popular posting techniques, like hashtags, @ replies and RT retweets. Twitter is also constantly re adjusting their data handling system so that people can see photos from Tumblr and other photo showing websites.
The site is banned in China, where they use Sina Weibo instead.
The site was temporarily banned in Indonesia (for less than a day) after Elon Musk's rebranding of the platform, because of the association of the symbol "X" with pornography.
Colbert Bump: A big part of why Twitter is so popular has to do with celebrities having taken to it so enthusiastically. And unlike most trendy things celebrities do, there's a (semi-)practical benefit to ordinary people following suit: People can now address their tweets directly to them and, supposedly, even get their attention this way.
Favstar, which keeps track of a user's favorited and retweeted tweets.
Twilog, which provides an online archive of your tweets and provides some useful statistics (like tweets sent on each day and each month, times you've mentioned a user). Although Twitter itself provides a tweet archive service, Twilog's archival services are in semi-real time, while official tweet archives have to be downloaded completely every time you want an updated version. Unfortunately, the site is in Japanese only, and you can't use it if your account is set to protected mode.
Favolog, from the same developer, keeps track of the tweets you've favorited. This is particularly useful in that Twitter does not provide any official favorite archive services.
Twiprofile, a service for creating an extended Twitter bio. In addition, it provides stats about your account, such as date and time of account creation (in Japan Standard Time) and average number of tweets per day, and you can add tags to yourself so like-interested users can follow you. Japanese-only, and has the unfortunate bug of splitting lines in mid-word, as the website is not meant to be used in English.