All ReadUps expire; the ReadUp creator sets the timeframe.
Why the limited time? We believe that not everything on the web needs to last forever. Sometimes you just want to have a conversation about something, fill your brain up, and move on. ReadUps are about the conversation happening NOW. In short, ReadUps are ephemeral.
ReadUps support discussion, and then they disappear. Your comments may float around in the ether afterwards (it's tied to the social network you're on, the paragraph, and a hashtag...meaning under the right conditions it could be seen by others after a ReadUp expires), but in their current form, as a part of a hosted ReadUp, they won't exist. The URL ceases to exist.