To everyone else having this problem you need to disable the proxy server settings. On a sony ericsson do this as follows:

Go to ‘Settings’ then go to the ‘Connectivity’ tab scroll down to ‘Internet Settings’ then go to ‘Internet Profiles’.
Highlight the profile you are using then press ‘More’ and select ‘Settings’. Scroll down to ‘Use Proxy’ and change this to ‘No’.

Now when you try to log into Facebook Mobile it will still ask for your user name and password but now it will accept it rather than take you back to the log on screen again and again and again and again and again and again and again.