I use FB once a day.
I treat it like a newspaper. A once-a-day update on friends lives that I poke at over a coffee, then throw away until the next day.
I find this a nice balance between having a "real" life, and maintaining some connections with folk that I otherwise wouldn't see socially very often.
Works for me. YMMV.
Once a day (e.g. 15 minutes) use probably gives a person 80% of the utility or value of 20x a day (e.g. 2 hours) usage. There is relatively little benefit from all those "extra visits", since almost nothing on Facebook is urgent, actionable, or going to cost money if not attended to immediately. (A great example of the 80-20 rule in action, btw.)
Then, ask yourself if $amount of money per month is still worth it for a 3G contract. This one really depends on your work and your environment (is there WiFi in walking range if you need it?). But it saved me time and money so far.