Jabber, I'm sure Eagle Heart is onto something: One more permutation to her suggestion is that your friend may have a Facebook account, but just not use it all that often. So they're not receiving your message.

I can think if three people who are close to me who have FB accounts, but they just don't use them.

While I believe FB notifies me via my email address when I have a private message on Facebook, I suspect that if I weren't regularly checking my FB account, I'd probably see that notice, think to myself that next time I'm on Facebook I will check for it, but then never actually get around viewing my Facebook account in a timely way. (And then, likely, forgetting it is there.)

In my experience, I have people I know who never reply to my email messages, but if I text them, I'll get a response back right away. Some of us just get into habits with regard to how we prefer to receive messages, and when they don't arrive in that format, we never actually receive them.

Explaining my thought here takes me back to all the tedious communications theory I had to study in my communications classes in college.

According to communications theory: [A message that is sent out, but is not received = an incomplete transaction.]

In my communications theory classes we were taught that we cannot automatically anticipate that a message sent out is a message received. There are just too many reasons why sent messages fail to be received.