A common misconception is that none must always be treated as singular. The customary support for this view is that none necessarily means "not one" (implying singularity); in fact, "none" is just as likely to imply "not any" (implying plurality). As noted in The American Heritage Dictionary: "the word has been used as both a singular and a plural noun from Old English onward. The plural usage appears in the King James Bible as well as the works of John Dryden and Edmund Burke and is widespread in the works of respectable writers today."
The most sensible rule is the one that governs similar words designating a portion of something (fractions, percentages, and indefinite pronnouns such as some, most, many, all, and more). Just as we write "some of it is" or "two-thirds of it is", we would write "none of it is"; just as write "some of them are" or "two-thirds of them are", we would write "none of them are."
Idiomatically, few of us would be comfortable with "None these people is happy" or "None of my friends is going with me." The sense here is plural: not any. Yet the myth of the singularity of none persists, even among people who frequently say, "None . . . are." (Why is it that some people cling to a simplistic rule, even when it's wrong, rather than face the necessity of making a choice based on sense?)
When the sense is plural (as indicated by a plural noun or pronoun in the following prepositional phrase – "none of [plural entity]"), none is plural; when the sense is singular (as indicated by a singular noun or pronoun in the following prepositional phrase – "none of [singular entity]"), none is singular.
Futhermore, we may have some instances in which either is correct. The American Heritage Dictionary notes: "The choice between a singular or plural verb depends on the desired effect. Both options are acceptable in this sentence: None of the conspirators has (or have) been brought to trial." And that is true because the sense of none may be construed here as either "not one" or "not any."
Two last points are worth noting. It is difficult to avoid treating none as a plural when it is modified by almost – "Almost none of the children were [not was] well-behaved." And, in constructions such as "None but a few of the students were able to complete the test," none must be treated as a plural.