Consider this classic XKCD: https://xkcd.com/810/
There are 2 possibilities: The AI can comment as well as a human, or it can't.
If it can comment as well as a human, our experience of using social networks will not be degraded. We still get the same experience as before.
If it can't, then its contributions won't get as many likes/upvotes, and they won't be particularly prominent. Most content users see will be human-created.
So I think the key question is whether AI will be able to manipulate the liking and voting systems. But there are methods for preventing this: ignore votes from accounts created after ChatGPT's release, or for a site like Twitter with paid accounts, ignore votes from unpaid accounts. It's not even clear what ChatGPT adds beyond a human voting ring, and social networks presumably have lots of experience dealing with voting rings already.