Actually, this is a real issue. For one, there is the Wayback Machine, which could very well see increased usage and mindshare as legally mandated content takedowns increase. For another, if, say, Facebook wanted to harvest data from this network to create shadow profiles and flesh out missing patterns in their analytics, then they could easily follow everything, keep the raw data/content internal, and never develop the ability to retroactively un-analyze that data when a delete request comes in.

This is a legitimate concern.

I would not put it past Facebook (or other businesses which are addicted to harvesting user data) to behave badly against networks like this. However, for the sake of their reputations they'd still probably do it quietly, which means many of the person-to-person attacks that deletion protects against would still be thwarted.

This is a problem the same way Facebook's privacy controls are a problem. Facebook themselves are not bound by them, but they're still useful if you want to protect your data from other users of the platform.

And FWIW, I think most of the big crawlers respect robots.txt - this is the same sort of thing.