Libraries should maintain the digital archive for their community.
Anything written on the web, by a member of their community, or about their community, should be part of the archive.
The question it should answer is this: What happened in this community in this period of time?
I've written a lot about this, under the heading of "future-safe archives." It's a real problem for the web. We are not creating anything like a permanent record.
This is probably the first piece.