Facebook does not optimize directly for time spent, it's too hard. I know because I worked on these algorithms. Time spent is a metric Facebook monitors and works to increase over time, but basically no work is done to increase time spent directly. Facebook also tries to give more reach to content that people like. There are whole teams of people that work on that, they are just less good at it than TT.

Tiktok optimizes for similar metrics. There's no difference there. The main difference is that they are more focused and have been doing short form video from the ground up.