GARNER’S MODERN ENGLISH USAGE
PRONOUNS
D. Indefinite Pronouns: Number. Traditionally, indefinite pronouns (anybody, anyone, etcetera) have been considered invariably singular. Indeed, as the subject of a verb, each of those terms must be singular…
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But, often, as in the following sentences [not shown], the sense undoubtedly carries the idea of plurality from an antecedent pronoun to a referent one…Try changing them to he, and you end up with deranged writing. SYNESIS dictates that logic trumps the strict rules of grammar with these words in transition…
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Although everybody and everyone carry the strongest suggestions of plurality, the other indefinite pronouns are almost as natural antecedents with they and them. That’s because they has increasingly moved toward singular senses. (See SEXISM (B).) Disturbing though these developments may seem to purists, they’re irreversible. And nothing that a grammarian says will change them.
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One thing you will learn if you pursue and interest in writing – in many cases the rules are sacrificed for good style. And that’s a rule.
But you have to look the thingamajig up in Garner’s or CMoS first. 😛