27-12-2011 12:13 PM
Yes, apps that use the internet will use data, likewise an email client which will connect to the internet to download or send emails.
Most apps like Twitter and Facebook aren't too data intensive and won't drain your allowance. A standard update or refresh won't involve it needing to download many megabytes (MB). Email clients generally don't use up too much data either although much will depend on how you've set it up and how often you're asking it to check the internet for new mail. Also,if you download a MB email then you'll need to use this amount of data to pull it to your phone.
The more intensive data usage comes with apps and clients that do things like stream live mobile TV given the amount of data needed (e..g live, constant images, sound etc).