I'll use two examples of how an un-replied-to FB private message affected my friendship. Let me know if my reaction is extreme!
I chose this path because my feelings were really hurt.
1. I worked somewhere for 14 years and recently left the job. It was volunteer work.
There were two paid workers there whom I liked very much, one especially - named "Marie." The other was "Ellen."
They were both FB Friends. I wrote a very short PM to each about how the person was one of my favorite people there, how I enjoyed working with them and thanks.
"Marie" wrote back with a wonderful short message.
"Ellen" appeared to have read it, and never wrote back, not even a "thank you."
I gave them plenty of time to respond.
Marie is still my FB Friend, Ellen is not - I deFriended Ellen.
2. "Jamie" used to be client of mine. She bought several handcrafted items from me. She told me she loved them. I took a beautiful photo of a thing I made that she had bought and sent it to her through FB private message. It was something she wore that she told me she loved. She had acted rude and hurried when she had last bought an item from me. She sent no reply to my private message. I later put her in a User group where she can't see my Wall but is not deFriended.
Maybe I expect too much? I reply to all FB PMs unless they are spam.