Would not say a lighthouse be the first "digital" message, blinking on and off (0/1) 
You could apply that same logic to an eclipse or star wobbles which have been happening for billions of years. Anything 'can be' digital information if you know how to interpret it, I guess this is the big difference from analogue, which 'is' actual usable information, where as digital data is interpreted as information.
Tribal drums were used to communicate over distance long before lighthouses, as were smoke signals. They could both be described as digital signals.
Surely the first signal was Genesis 1:4 "And God saw the light, that it was /bad. ]