It's actually possible to evoke some interesting responses from the algorithms by reducing the amount of data input.

For example: When I set up a new facebook account for my mother (at her explicit wish), she had no friends or interests marked yet. Facebook showed her some random ads and posts.

During the setup I was scrolling through her timeline and my phone beeped so I stopped scrolling for about 2 seconds. The post shown was a random post about some fish.

When I picked it up, I saw it quickly replacing the next random post with something about the same kind of fish. So evidently it even looks at how long you look at certain content to determine your interests.

I suppose it is possible to derive other algorithmic determinations using similar methods.