What have they done that is illegal?
The sharing of personal data which is probably embedded somewhere in the hundreds and hundreds of lines of their Terms and Conditions of use.
There may well be potential issues regarding where the data has been shared to if outside the EU or larger EEA area.
If something illegal has happened then Facebook or Cambridge Analytics deserve to be held accountable however if data has been harvested in line with agreed Terms and Conditions and now people are violated and offended as they don't like why the data was used then fúck them.
Remarkable stupidity of the people to think they were handing over all this data to a global giant who ask for no financial donation, and to then think said data was not being used for x, y and z.