I remember here in the UK, Three (a UK mobile network operator) allowed for free access to Facebook. This was before the days of the iPhone though and it took you to a stripped down mobile site.
Three also offered free access to Skype. It used (uses?) a Three-specific Skype client (available for many, but not all, models of phone) to do the presence and signalling over data, and the voice over the normal mobile voice channel:
