Welcome to the forums! Either data plan that you would get with your BB device is truly UNLIMITED for email and web browsing. The 5GB limit would only come into play if you were using the device to tether to a laptop. As for the Facebook 1.6 questions, I'll have to defer to others as I'm not a social networking guy. I think you'll really enjoy the Tour....I've had mine about a month now. Good luck!!
Email and Web access are under two different plans, both of which are unlimited.
The Cheaper plan is for people the will be connecting their email through the Blackberry Internet Service (BIS) This one handles all web based email like yahoo, gmail, msn, etc. and will also do corporate email. But it does get web and it does not sync wirelessly for contacts and such, just email and web access.
The more expensive plan is for Corporate Users that have a Blackberry Enterprise Server (BES) running. It again is unlimited web and email.
Some will have mulitple emails on one berry. I have that. I have my corporate email plus a personal account. I do all mine through the BIS.
The 5gig limit I think you are referring to is for "Tethering" This is when you connect you berry to your laptop for internet access on the laptop using the berry as the modem. This is an extra fee.
Also SMS, MMS are both extra since they actually go through the voice part of the phone.
I hope that answers everything for you.