Customers of O2's Mobile Broadband (3G, HSPA) service are currently unable to access the Facebook social networking website. It's not known why this problem is occurring although the operator is aware of it (o2 forum post).

Thankfully there is at least a temporary solution, which involves using the restrictive iPhone based Facebook app interface URL as a workaround: http://www.iPhone.facebook.com . Not ideal but it's better than nothing.

UPDATE - 08.25am 18/02/09:

The problem has been fixed.