I noticed since a few weeks that the rate of visitors/day has dropped down in an irrealistic manner. It went from about 5000 visitors/day to 5.
Did something change in the way it is counted or is it just a bug ?
This is a bug. The response from Stack Exchange so far:
We're investigating, we've had a number of large changes to our Google Analytics infrastructure, looks like we missed some bits...
Area 51 still looks broken however the network overview page seems to be working again (currently showing 4.5k/day).