Does Netflix detect fraud on streaming accounts?

I have a Netflix streaming account and am not trying to commit fraud. But I know at least one person who was given a friend/relative’s login so he could watch for free. I would think this is common. Does Netflix try to detect this and prevent it? Seems like it would be a big source of revenue leakage.

They actually do allow up to a certain amount of devices (4?) to stream at the same time. I know they’re toying with the idea of having a cheaper plan that will only stream to one device at a time.
I’m not sure what the rules say WRT to who those 4 people can be.

I just checked my plan. For $7.99 I can stream to 2 screens at a time, for $11.99 I can stream to 4 screens at a time.
My very brief looking didn’t imply that I had to own both of those screens.

I get the feeling that’s why they’re going to go to another plan that only allows 1 screen. Personally, I’ll jump right down to that one screen plan, then if a friend ever asks me if they can use my account I’ll be more likely to say “Sorry, I can only stream to one device” instead of “Sure, let me just go upgrade my account”. Then their more likely to get their own account. Even though all we would have to do is not stream at that same time.