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If you look at the file history of the patent application (which you can get from http://portal.uspto.gov/external/portal/pair), you will see that the patent was allowed only after adding the "where the link enables a viewing user to participate in the same activity" limitation. It is mentioned in the Interview Summary.

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This is an excellent resource that I was unaware of. That limitation was not present in the 8-19-2009 claims, but was in 12-16-2009. I made a cursory glance over the documents available on PAIR, but was unable to find the examiners specific rejection that caused them to add this. There are a lot of references on this application, tho.

Look at the 12/14/2009 interview summary (an interview is where the patent applicants talk to the examiner), and in particular the third page.

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