It's very responsible for building the hate echo chambers, though.
Some people don't. Social media is a mirror.
People aren't zombies - you're not powerless before an omnipotent algorithm.
You have to make a deliberate effort to not interact with polarizing content. Facebook constantly puts poisoned bait in your feed in the hopes you get hooked. Happy people don't engage as much as angry people. Seniors who don't have an IT background, children who don't have enough common sense, poor people who don't have enough education to know Facebook tracks 50K+ traits to better manipulate your behavior, etc.
It looks like you're not considering all the different kinds of users of Facebook in all countries where it operates. You can start looking at funny memes and slowly end up with paid divisive government propaganda.
Social media exploits human behavior, often in the worst ways. You say people aren't zombies, but people aren't rational actors, either, they are people. Human behavior lies somewhere in between zombie and rational actor, rarely completely at one extreme. This is where the responsibility of a megacorp controlling the world's largest social media platform comes in. People can have their behavior manipulated on these platforms, and advertising is the proof - why would advertisers spend if it didn't work?
Social media is definitely more than a mirror, and FB deserves the general negative sentiment it gets because it consistently fails to self regulate in the interest of its users or the general public. FB has two objective functions due to its corporate structure: 1) shareholder profit; 2) Zuckerberg's agenda (currently the Metaverse, previously user growth).