A system is kind of like an unthinking lifeform. It will adapt to find ways to make itself stronger and to grow. It will use whatever resources it has access to to and does not care about (or is even conscious of) the damage it does in the process. All systems are like a virus in this way. Social media as a system is just playing this out. The social media system feeds on engagement and use and, it turns out, that isolation and polarization lead to more social media engagement. So it adapts itself to make more that, the users well-being is irrelevant. Corporate bureaucracy, capitalism, religion even are similar. They may have been created for some purpose (that was useful for the people who created it), but once they reach a certain scale, that purpose inevitably shifts to simple self-perpetuation.
Another example is a charity formed to help with some problem which it initially does well at. But then over time the charity matures and more and more of its funds are devoted to keeping the charity running, paying salaries, renting office space, paying for trips, organizing fund-raising events. Less goes to the cause. The primary purpose of all the fundraising then becomes to cover the overhead of doing all the fundraising. The cause is a second order consideration at best.
Social media has matured from being about connection, information exchange, community building. Now its just to keeping the clicks and comments coming so the ad-revenue and data-mining can be used to cover the cost of the servers, developers, and executive bonuses.