grammatical number - Is two-thirds plural?

Is 2/3 always, sometimes or never plural?

E.g.

1a) 2/3 of the pizza were eaten.
1b) 2/3 of the pizza was eaten.

2a) 2/3 of the visitors were men.
2b) 2/3 of the visitors was men.

I feel that example 1 could go either way but example 2b sounds very wrong.

Is there a rule for this?

Answer

It depends on whether two-thirds (or any similar proportion) is regarded as a measure of amount or of number. In (1), the emphasis is likely to be on the amount of pizza eaten, and not on the number of individual thirds, so (b) would be appropriate. In contrast, in (2) the emphasis is on the number of visitors who were men, so plural concord, as in (a), is required.