In formal English grammar, the pronoun "they" is always plural. There is no gender-nonspecific singular pronoun in English. This is a problem, for there is often a need to express such a pronoun.
There are alternatives. In graduate school, the use of gender-inclusive language was pounded into me, so I now reflexively use "he or she" or "s/he" or "his or her", even when writing for Neatorama. But it is awkward and some writers prefer the smoother if gramatically icky singular "they". Jenn Doll of The Atlantic is not among them. She wants English speakers and writers to use "he" and "she":
Let's talk about something. Let's talk about the "singular" they. That's when a writer or a speaker — a he or a she — is discussing someone who might be either a he or a she (it's unknown, or the writer doesn't intend to make a subject or object gender-specific and instead hopes to convey a universality of personhood). So instead of writing, say, he or she did x or y, the writer uses they. It's everywhere, proliferating like fruit flies 'round a bowl of rotting bananas, bad writing surrounding bad writing. [...]
I'm all for a certain flexibility and adaptive ease with regard to language and how we use it. I'm happy to add three exclamation points to a sentence or write in ALL CAPS when it seems to fit the moment, especially online. But I see absolutely no reason other than laziness to start subbing our hes and shes with a clunky they, or our hises and hers with theirs. There is a reason we have distinct pronouns, and that is so we can be specific. If we don't know the specifics, we should try to find them out, or use one of those handy words — he or she or one, for instance — that get around the they problem. Peppering one's sentences with some hes and shes can be kind of nice, really, a way to assemble a collection of characters who are certainly more real and individualized than a collective they.
There is criticism that the use of he as the generic pronoun is an example of linguistic sexism of a sort, and I agree there's no need to always use he as the default if you don't know the gender of the person about whom you are speaking, or if you're using the pronoun to stand for persons of either gender. You can just as easily swap in a she; mix it up! Make it fun! Keep people on their toes! Maybe even create a new word, and make it happen! The message that something should be easy, that we all understand anyway, that it doesn't really matter and we should give up the fight may be the most galling part of this argument, though. Since when was writing or creating art with words (if you're being high-minded) supposed to be convenient? Since when was past history the rule for how we live in the present and future? Break the rules if you must, for a purpose, to make an impact. Don't do something because it's easy and everyone else is doing it. If a word sounds like it's landing with a horrid thump in your ear, it's landing that way to at least some of your readers. Every time I see a singular they, my inner grammatical spirit aches.
UPDATE: In the comments, Gerard Van der Leun correctly points out that I forgot to include in the poll the traditional use of "he" is an implied gender nonspecific pronoun. I've now added it.