Facebook was awesome when it was the only social network; now we have Instagram for social image sharing, WhatsApp for messaging, Snapchat for ephemeral messaging, YouTube for social video and TikTok for social short video plus Discord and Twitch for social gaming. Even Facebook's social casual gaming moved to mobile gaming. Facebook wanted to be the everything social network just like Amazon is the everything store…
They got greedy. Word got out about how they'd manipulate the algorithm to get you to spend hours per day there, how aggressively they mined and sold your data, etc. It became unfashionable to use. People who had spent a decade recording their lives and connecting with their friends walked away from their accounts.
In the midst of all that, they bought Instagram. I've recently moved to NYC; paradoxically, people here ask for your Instagram the way that people used to ask for your Facebook.
Social networking isn't quite as consolidated in Meta as dating is in IAC/Match, but it's close.