Can people tell if I am viewing their Facebook profile?

[Finn. :)]

No, they cant tell if you are viewing their profile. If they changed their account to private then it would be just because they wanted to, not because of anything you or anyone else had done viewing their profile wise.
It would have been a coincidence that more than one person made their profiles private when you had looked at them previously.

:]

[Anna N]

No, it's completely private. They can never tell.
Many people simply aren't aware of how much other people can see (people who aren't their friends), and when they find out they change their settings. Also sometimes somehting happens to cause them to change things to be more provate (ie a break up, applying for a job, or simply discovering strange people look at their profile and/or add them).

Your Facebook stalking is safely secret. Everyone does it to an extent. And it includes people you're friends with--we're all guilty!