All of the answers thus far have been very vague and abstract. New social networks generally arise and gain traction when they enable a new form of content creation that’s vastly more compelling than what exists before that point for at least one core set of users and then expands from there as network effects kick in and more use cases are discovered by users.
In the cases of Twitter and Tiktok specifically, Twitter’s 140 character limit and real time nature forced people to be succinct and constantly up to date which forced a cryptic, humorous, deeply in the know type of content that made it the “cool kids” hangout spot for, IIRC, the burning man community which spread it from there.
Before Tiktok, it was very relatively easy for the average person to make compelling photographs but there was still a big skill jump for people to make compelling video content that strangers would want to watch (as opposed to friends which snapchat was focused on). Tiktok’s innovation was realizing that pairing video with music drastically reduced the skill ceiling of making video content others wanted to watch since all you had to do was to do a goofy dance.
Instagram figured out how to make crappy cellphone camera photos look interesting via filters, Facebook figured out how to keep up to date on the status of your weak ties via a feed, Snapchat figured out how to let people send nudes to each other (relatively) safely. Reddit figured out how to do a Fark style list of links but collaboratively filtered instead of human curated. Pinterest allowed Mormon housewives to share home decorating and fashion photos with each other.
It’s very fashionable to pooh pooh the next social network because if you’re not the target user, then it all looks very silly but all of them gained traction because the content experience was simply orders of magnitude more compelling than the next best option for at least one group of people.