Except the following:
Since I use prepaid I'm not connected to a carrier (the carrier does not support BIS)
Mostly of the time I use my blackberry on Wifi-networks
Recently however I noticed that for example applications like facebook don't connect via the Wifi...
Is there any way to get that application to connect via Wifi?
Also are there any applications that connect using the wifi connection available for Wifi?
There are a small number of data applications that can use WiFi, but not many. Look in the How To forum section, and you will find a long thread dedicated to how the WiFi can work.