You purchase a data plan based on what you have historically or think you will use. Since FaceTime over cellular is "data" it will eat into that allotment. I just switched my plan to the shared plan as my most of my usage occurs in wifi areas...will simply track my usage closely so I can change the plan before any overage charges occur. It's a good idea when there are multiple users on a single account. Switching this month already saved me $20 a month versus the lowest cost family plan.
But unlimited voice and texting on the share plan is key with the wife and kids...data is secondary due to wifi being available for us usually everywhere we usually use it.