A number of them will be being used by search engines (such as google) to update their indexes of pprune. It is no surprise since pprune often appears high up on google searches... Google is also often quicker/easier (in my opinion) to find a long lost thread than the built in search tools. They wont be re-posting posts though (unless their design is flawed).
My memory is that you need to create an account to post here which bots have great difficulty doing. Could it be there are some pilots who are just a little too fatigued to function properly logging in?