Please explain why facebook is so addictive some people & they are reluctant to deactivate/delete their?

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The sheer fact that you said "I think" and "in a couple weeks" probably answers your question. You're not sure enough about it to go by impulse right now, and you have people who it helps you stay connected with and things you want to be able to access or find out. None of that is meant negatively. I'm not implying you're remotely addicted to Facebook, but you must find something positive in it or you'd just announce you're leaving and do it.

To answer your question - to most people Facebook is addictive because:
a) they like to know what their friends are doing
b) they like to tell people what they're doing
c) they need an "idle process" (something to do by reflex when bored and with internet access)
d) they want to keep up with news from certain groups and icons
e) they want access to Facebook-exclusive competitions and games
f) it's simply considered more sociable than anything else on the internet, so they can enjoy casual gaming, newsreading, bitching, spying, and timewasting, without being considered antisocial - for the same reason they may be on Facebook just because "everyone else" is.

Hope some of that helps