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.
The whole point of the preprint repositories is to provide an outlet for new results that bypasses the peer review system without offering an alternative to it. Since preprints are not peer-reviewed, they should not be cited (otherwise the whole peer review system would be broken, and we might as well cite Wikipedia). As they are not cited, they cannot contribute to an impact factor.
The most a preprint can receive in terms of "impact" is exposure via social media, and most preprint archives offer such metrics (bioarxiv for instance). This can be nice to look at, but ultimately it really means very little about the traditional impact of the paper, the same way that a newspaper writing about some research does not increase its credibility or scientific importance. It can also give a false sense of achievement.
From my experience the media hype around preprints is not related to the repository but rather the authors sharing it via twitter and it being picked up by sufficiently connected users. Again, this doesn't say anything about the impact of the paper but rather how "viral" it is.
Why are you publishing a preprint? What would be your motivation to choose one repository over another? I can't see any reason not to go with whatever repository is respected in your field. If you are publishing a preprint to avoid scooping, or out of sheer altruism, it shouldn't matter so long as the information is publicly available. If you want publicity, it also doesn't matter (as explained above).