I think a better analogy than a library would be your average day in the office: would you want everything you say and do in the office recorded for eternity? Sure it would help, say, catch fraudsters, track responsibility and credit, allow sociologists fascinating analysis - but is that worth it? The >1GB of Google+ is a good example. Probably many interesting posts from people that are the core experts on topic X - and many nonsensical Twitter-like posts of people sharing whatever they encountered or thought that day.