The same way people handle Word, Excel, PowerPoint, Outlook, and OneNote. They fulfill different functions to different people. I could do notes, make spreadsheets, or produce presentations in any one of those apps, but each task is more or less practical in each tool.
I haven't used social media in a while, but when I did they fulfilled these separate functions:
Facebook - Keeping up to data with family (or anyone over the age of 40). Communicating with Social groups/organizing events. Messaging friends and family with Messenger.
Instagram - Maintaining a photo diary of my life. Sharing what I'm doing currently with friends. Following celebrities. Messaging.
Twitter - Following celebrities, but textually rather than with video stories. Keeping up to date on current events and trends.
Snapchat - To be honest, Instagram stole SnapChats cake completely for me when they made Stories. Younger generations use it more from my experience. Used it largely for messaging without a paper trail.
TikTok - Entertainment and following social trends. It's like YouTube, but with a better recommendation algorithm and shorter form videos.