Facebook was just a way to talk to your friends and replaced Yahoo! Messenger, except that you could find new friends your age in your area as well. This worked because the rest of society wasn't aware of Facebook. LinkedIn might be a better candidate for a personal directory.

Twitter and Instagram are both ways to connect to people you do not know, but the former is genuinely about finding people to which you share some interest, whereas the former is still mostly about adding friends that you already know. Instagram replaced most of the functions of Facebook that I described in the first paragraph.

TikTok is basically the video version of Twitter with a good algorithm that finds new content you might like.