Molarity = mol/L
Assuming it's a solution in water, 1 ppb = 1 g/1,000,000,000 mL (remember, 1 mL water = 1 g water) or 1 g/(1 x 10^6 L) which is equivalent to 1 ug/L
So, to get to ppb, first convert to mol/L:
50 uM = (50x10^-6) M = (50x10^-6) mol/L
Convert to grams:
(50x10^-6) mol/L x (molecular mass) = g/L
Convert to ug/L by dividing by 1x10^6
By the above, ug/L = ppb
It's been a while since I've done gen chem, so someone else might want to verify that. You will also have to forgive my poor use of "correct" exponential notation.