The "Chat" function on Facebook is simultaneously annoying and creepy at once.
You are effortlessly and mindlessly skimming over the waters of FB Nation when, out of the blue, you hear that "pop" and see the chat window... "What's up?"
"Well, I was mindlessly surfing Facebook (and now I'm forced to drop everything in order to engage and respond to your hopelessly open-ended query in a manner that would indicate that I'm pleased to have this opportunity to interact with you."
The "chat" function on Facebook, is the cyber-equivalent of "dropping-by" a friend's house un-announced. It could turn out to be a really welcome surprise but it is still a surprise none the less.
Best not to take any chances. All things being equal, call or email first.