I have a 500MB plan with Telecom New Zealand, and before using Windows Phone 7 I had a Windows Mobile 6.5 device.
I don't have push email enabled, and only manually update it early in the morning and a couple of times a week in the evening when I am out.
I don't have my Facebook account loaded anymore because I don't really need all the contacts there, and most of my short updates go to Twitter.
I do have a Twitter client, which again is manually updated, and only when I am out.
Interestingly enough though I am always getting closer to the 500MB limit, more than ever before. I have the location update turned off, and all Marketplace actions are done over WiFi at home or when plugged to my PC.
With Windows Phone Mango I suspect I would need to actually bump the plan to the next level.
Is anyone else seeing this?