I've had several hours long chats with people I have never met personally before--some who are very forthcoming with personal information and it seems as if we have been friends for years when in fact we have only met on Facebook. A little odd, but then at the same time, its a real person with real feelings and I try my best to take people at "face" value--- or should that be "Facebook value.
The bolded is why I stopped doing any form of internet chat, except with people I have a close relationship
outside of the chat, or for the purposes of work. It's an interesting phenomenon, but online chat (in particular) creates a false sense of intimacy. I've been hurt by this, several times, and so stopped participating. While it's nice to take them at face value, please understand that they could be the absolute opposite of what they claim to be. The old cartoon "On the Internet, nobody knows that you're a dog" is absolutely true.
There are a number of reasons why someone won't reply:
* They don't feel like chatting with you. They don't feel the relationship
is nearly as close as you do.
* They aren't at their computer. It's quite possible to look like you're logged in, while nowhere near your computer.
* It's their cousin/uncle/roommate/cat logged in and not the person you think that it is. They don't know you so they don't respond.
* Your IMs are getting lost in the great mass inter-tubes.