Impact factors of preprint repositories I'm wondering if the growing catalogue of preprint repositories have known 'impact factors' similar to those of popular print journals. If so, can anyone highlight them?
Even if there are services computing them, the impact factor for a preprint repository would not be useful.

Impact factors represent how many citations an article gets on average within a certain period of time. As such, they are supposed to be a rough metric for the selectiveness of a journal. If a journal is highly selective, then only highly impactful articles should (in theory) make it in, driving the impact factor of the journal up. 

Now a preprint repository is explicitly designed to not be selective - Every paper gets in (possibly under some conditions that are hardly comparable to peer-review). As such, the impact factor of such a repository would not measure anything of interest.