I always just assumed that they were using it to weight web page rankings, and that it probably paid off just for that reason alone.
If a website has a lot of genuine RSS subscribers in Google Reader who read it regularly, then that shows a strong degree of trust in the source, and thus a good reason to rank it highly.
You can't really mine social networking / emails for the same kind of information. Not unless everybody who used google reader started using G+ the same way (which is not gonna happen).
Wow, that's a good point though. Does anyone know whether Google uses links in private emails to refine search rankings? With gmail being the most popular free email by far, that might be a small but significant edge on the competition. (Not that Google needs such an edge at the moment, but that doesn't mean they'd pass it up.)
If a website has a lot of genuine RSS subscribers in Google Reader who read it regularly, then that shows a strong degree of trust in the source, and thus a good reason to rank it highly.
You can't really mine social networking / emails for the same kind of information. Not unless everybody who used google reader started using G+ the same way (which is not gonna happen).