error with apps over mobile internet on Hero

using a Hero, sim free.
its running android 2.1
im on vodafone.
with wifi enabled, i can access facebook using the official app fine.

if i enable mobile internet, i can connect normally to the internet using opera, can access twitter, but when i try and use the facebook app, i get the following error:

unsupported content type text vnd.wap.wml

anyone any idea what that is?