Yes, mostly just analytics and standard fare from all websites. For the most part, it's just the regular "Share on Facebook/Twitter/Etc." buttons that you see all over the internet. All of the social media have trackers embedded in those buttons. It's always confused my why anyone would actually share what porn video they were watching with their friends on social media.

The one thing that isn't standard for porn sites is ad serving/tracking. They usually don't use ads served from Google and Facebook, probably because most people don't want their "regular" ads being served on a porn site. So most major porn sites have their own ad system.

Second-author here. Details are in the paper, but yes, Google Analytics is highly prevalent. Google doesn't allow porn sites to use their ad networks so we didn't find much Double-click, but it is fairly hypocritical to say "we won't show ads on porn sites" while also having no problem allowing porn sites to install your tracking software.

I guess you can add browser fingerprinting, etc. which most folks seem to do these days, since there is a cat-and-mouse game of users slowly becoming more privacy sensitive and using basic tools like incognito mode and ad blockers.