How do unique results per user work on huge sites like Twitter, Reddit, etc. Especially with complex criteria.

How do sites with millions of users that serve up unique search results weighted by the individuals interests in a quick and scalable way.

For instance let’s say I have a site with 50,000,000 users, 100,000 categories, 10 different ordering criteria (new, best, controversial, etc). I want to return back the results per the persons unique criteria updating continually on the fly showing the best content from all the areas ordered by the best first.

Really interested in how you’d go about doing something like this at huge scale