1. arXiv, ~21k/month
2. bioRxiv, ~5k/month
3. medRxiv, ~1.3k/month
It would be interesting to see usage of other _xiv's and other pre-print publishers!
[1] https://arxiv.org/stats/monthly_submissions [2] https://api.biorxiv.org/reports/content_summary [3] https://api.medrxiv.org/reports/content_summary