Have found it near impossible to find any stats around this that aren't 5+ years old
With about 50k users about 14.8k are single sign on, with the majority being google authentication. Something I noticed was that early on we got way more Twitter sign ups, but over the last 2 years that seems to have changed and we get a lot more email sign ups.
I’m not a fan of single sign on for most of the services I use every day like Spotify, but for throw away or single use signups it’s great. I’ve definitely noticed a trend where other people do the same so I expect the percentage to fairly pretty greatly depending on the service.