Some profiles have a "Chat Now" link on the Action block, some don't. what's the deal?

If a member is online you will see the "Chat Now" link, which will open an Instant Messenger application. The other member will receive a pop-up invitation to chat. Once they accept, you can chat in real-time. It you have a web cam you can share video. If you have a microphone you can also talk. If a member is not online you won't see the "Chat Now" link.

Related Topics:
What's the difference between the "Chat Now" link in the Action block and the "Chat" button on the main menu?
What are all the ways I can communicate with other members?