If your partens want new iPhones and have unlimited right now, they keep unlimited. If you wanted to be added to their plan, you wouldn't be able to get unlimited because you are a new customer to the add a line.
Data plans start at $15 for 200 MB and go up to $25 for 2GB of data. 60% of customers don't go over 200MB a month and 98% don't go over 2GB.
Remember... I can't stress this enought, you can use WIFI as much as you want and it doesn't count towards your 200MB or 2GB. So if you have wifi at home and work you can use the internet as much as you want.
Facebook doesn't really use up a lot of data and IM is even lighter. IM uses very little since its sending basic text messages.
So, if you would like to find out how much data you use, use this Data Calculator provided by AT&T:
http://www.att.com/standalone/data-calculator/index.html
Good luck and enjoy your iPhone if you get one