Unknown people on my Facebook friends list?

[winterbunny]

I was looking through my friends list and there were several people, more than 10, who I know for certain I've heard of or added - and all of them had deactivated pages. I removed them but is there any way to stop this from happening? Has this happened to anyone else? I've never had any problems with anyone hijacking or planting a virus on my page.
What I'm saying is, I never added these people in the first place. I *know* people can deactivate their own page - but that isn't the question. The question is, how does someone get on my friends list when they never sent me a request and I never added them??? That's why I said they are *unknown* -
I don't know them and I never added them.
ETA: I meant to say *never* heard of - hence the reason I called them "unknown people" in the question.