Impact factors of preprint repositories
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.