I was on Facebook, then deleted my account, then rejoined it, though I still hate it. These are my reasons, which I posted prior to leaving:
I disagree with the way Facebook operates, i.e. one entity controlling social connection information, and existing through advertising. Social connection information should be decentralized, i.e. through websites (such as blogs) with feeds. Facebook has also made it clear that they have no problem sharing your information with others by default.
Facebook gains power when everyone is on Facebook, even when those people don't contribute. If all your friends are on Facebook, you may start to treat it as a regular means of communication with people - the same way you can treat telephones as a regular means of communication because everybody has a phone number. The problem with Facebook replacing phones and email is not just that it's a poorer form of communication - it's that they're a company, while phones and email are not in the control of one company. By quitting Facebook, I help confound their efforts to make Facebook such a regular means of communication.
The fact that it stores connections to your friends puts those friends at risk of extra information being known about them, and puts you at risk from them, even when you're not doing anything. If someone links something to you, Facebook can infer some connection between you and that person. Risking my friends' privacy is much worse than risking my own.
My being on Facebook has possibly implicitly convinced people that Facebook is safe or has no privacy concerns. "Hey, look - that guy knows a lot about computers, and he's on Facebook, so there must be no problem with it". It is wrong of me to act as such a potential role model, because there are indeed privacy and security problems with Facebook.
Additionally, in recent years (since about 2022 or so) I've found that Facebook has gotten extremely bad and developed extra problems in addition to the above:
The main feed has a huge proportion of ads, even with an adblocker, and a lot of things get shown to the user which might not technically be ads but which have an ad-like or promotional aura to them. This makes it very difficult to access the needed information, i.e. posts for in-person events that I want to go to, without being bombarded by this irrelevant information. But the need for the relevant information gives me the bad habit of continuing to come back and scroll through my feed looking for it, thus getting sucked in to being coerced by the unwanted information and posts, and that's how Facebook engineers it to control us and profit from us.
Its algorithm has contributed to the spread of misinformation or ideologies by selling the ability to share misinformation / ideologies, which is attention-grabbing, to advertisers ("customers"). This has affected what people believe and the method by which they arrive at belief in a negative way.