How is that some people are offline,yet reply me soon after my message?

Sensative notification settings? No messenges them so if anyone does their phone e mail fb myspace and sms all send a notifucation. The hermit of notification scenario.

I may switch to another tab on my browser but get notified when I get a message and I switch back to SW tab to reply.

Go to your settings look at your privacy settings

Not everyone has the green light enabled and they may be getting notifications on their phone