Please post a link to the orrigional website with the supporting text.
You need both voltage and current measurements to get the power factor.
Oh, I've just realised that this circuit does that, because it uses silly yanky squiggles for resistors I thought T2 was a resistor and it's not, it's a current transormer. I assume it consists of a number of turns of wire wrapped round one of the mains conductors. The number of turns will depend on the current you want to measure. There should also be a burden resistor which is mising so in this case the output voltage will depend on the input impedance of the 339 which is very high and very variable. I doubt it really matters as the L339 is only acting as a zero crossing detector, I don't know about 100 turns might do.