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.