What does upgrade your data service plan mean?

[Bapestagal:p]

Whenever I try to go on fb or twitter it just says you need to upgrade your data service plan but what does that mean and how can I do it without going to an at&t also I have a blackberry bold 9000

[AL]

Your data plan is your contract with AT&T: What you pay for, and what you are allowed to do. If you wish to do more, AT&T required you to change "plans" to a different plan that allows or facilitates what you want to do.

Example: You signed up with AT&T for "voice only" phone service. Now you want to be able to "text" also. You cannot text on your current plan, or for the amount you currently pay per month. You must "upgrade" ( sign up for a more expensive, plan that allows you to do more )