It looks like Pixiv just has a "people who favorited this post also favorited these other posts" type system. I messed around with something like this for Danbooru a long time ago. My results weren't very encouraging. For posts that have only a few favorites, which accounts for a huge number of posts here, you don't have enough data to make meaningful recommendations. For posts that have a lot of favorites you tend to just get recommendations for the lowest common denominator — stuff like tony taka or ishikei posts that everyone favorites and that you probably already know about.
Overall generating decent recommendations is not easy to do. Not to mention that the algorithms used tend to be slow and the site is already struggling with speed issues.