Most social media that got popular took something that people were already doing and brought it to a wider audience. Here are some examples:
Myspace
In the late 1990s and early 2000s, it was common for people, especially teenagers to make personal sites introducing themselves and linking to a few of their friends. Sometimes they would add life updates to the sites, but often linked to a blog hosted elsewhere (e.g. Livejournal). Myspace just institutionalized that.
At first, it was pretty much Myspace for adults, then it integrated something a lot like an internal RSS reader.
Flickr
As digital cameras became popular, people started putting photo albums on their personal sites. Flickr made it easy.
A blog and feed reader combination you could post to by SMS at a time when most phones didn't have internet.
Image-focused Twitter designed to be used from a smartphone.
A then-popular social bookmarking site called Delicious had a "popular" section for links many people bookmarked. Instead of people having to find the links independently, Reddit added vote arrows. Eventually they added comments and it became a forum, which is also a thing people were doing already.
Youtube
This is the exception. Hosting videos was ridiculously expensive in 2005 so people weren't doing it much. Youtube set VC money on fire until Google bought it and set Google money on fire for a decade or so until it finally started to be profitable.