Calculation of maximum input impedance
As long as U1 is active and the output is stable (which may be negatively affected by the C2), the impedance looking into Vdiv is 2.2K || 990K || 100nF.
At 50Hz, approximately 2.2K.

As @Neil_UK suggests, maybe consider a circuit more like this one:



simulate this circuit &ndash; Schematic created using CircuitLab
Make sure the maximum peak voltage divided by the 990K does not exceed the minimum current draw of whatever is connected to the +5V supply unless your regulator can sink current (most can't). If necessary, you can add a 5mA power LED or whatever. Otherwise, a positive transient could lift the +5V rail and possibly damage something.