Yes and no. Yes, when you're talking about multiples of fractions eg. two thirds they are plural. But when you're talking about a portion of a single item, that item is singular.
One third of the pizza was eaten. (part of one pizza was eaten)
Two thirds of the pizza was eaten. (part of one pizza was eaten)
The subject is pizza - singular. If you have one third of a pizza, and I have one third of that same pizza we have two thirds (plural) of one pizza (singular).
Of course this changes when the subject becomes plural:
One third of two pizzas were eaten. (part of two pizzas were eaten)
One third of the visitors were men. (multiple visitors were men)
Two thirds of the visitors were men. (multiple visitors were men)
The subject in this case - men - is plural.
The sentence doesn't make sense if the subject is singular (Two thirds of the visitors were man). I'll just change the meaning slightly so it works:
One third of the man was on fire. - Singular
Two thirds of the man was on fire. - Still singular
Clear as mud.