So, if anybody later also will use it, this is the solution.
First, fetch the page id from FBgraph, in my example: modelairfields:
https://graph.facebook.com/modelairfields
This gives me the ID: 257995927658591
The Code for B4A:
Try
Dim In As Intent
In.Initialize(In.ACTION_VIEW, "fb://profile/257995927658591")
StartActivity(In)
Catch
Dim p As PhoneIntents
StartActivity(p.OpenBrowser("https://facebook.com/modelairfields"))
End Try
It starts the Facebook app, if not installed, it starts the browser.