Xjyuk

This question might sound a bit facetious but I have been wondering for a long time. Many western pop songs are addressed to an anonymous "you". Presumably the song writer wants to write a song that could be about anyone and not a particular person, and I do get that. But I was wondering at what point in the history of western popular music did this become common?

I can think of at least a World War 2 example ("I'll be home for Christmas") in which "you" appears (in the lyrics "you can count on me") but I expect it was long before that.

Was there a particular point where "you" became common, and if so, when? If not, was it always common?

Odes to someone unspecified by name (if not by profession or by pulchritude) date at least to 1914. Titles of World War I songs include "Oh, You Beautiful Doll," "Your King and Country Wants You," "Don't Bite the Hand That's Feeding You," "Pack Up Your Troubles," "Keep the Home Fires Burning."

Odes in general go back to ancient Greece and have been composed continually since then. Some of those too omit the addressee's name, for instance
Schubert's 1828 song Der Doppelgänger.

As for when this became common, you'd have to run some statistical analysis of a corpus of song lyrics, and look for a knee in the curve. Maybe flag lyrics that have plenty of "you" and an absence of proper names.

I'm going to go with "it was always common". Certainly many Renaissance madrigals are written to "you" (or "thee", as the case may be):

Come again!
Sweet love doth now invite
Thy graces that refrain 
To do me due delight.

To see, to hear,
To touch, to kiss,
To die with thee again 
in sweetest sympathy.

Come Again, John Dowland, c.1597

Alas, my love, you do me wrong,
to cast me off discourteously;
and I have loved you so long,
delighting in your company.

I have been ready at your hand 
To grant whatever you would crave
I have wagered both life and land
Your love and goodwill for to have.

Greensleeves, Unknown c. 1580

And even the oldest love songs/poems are written this way:

Behold, you are beautiful, my love. 
Behold, you are beautiful. 
Your eyes are doves behind your veil. 
Your hair is as a flock of goats, that descend from Mount Gilead.
Your teeth are like a newly shorn flock, which have come up from the washing, 
where every one of them has twins. None is bereaved among them.
Your lips are like scarlet thread. 
Your mouth is lovely. 
Your temples are like a piece of a pomegranate behind your veil.
Your neck is like David's tower built for an armory, 
whereon a thousand shields hang, all the shields of the mighty men.
Your two breasts are like two fawns that are twins of a roe, which feed among the lilies.
Until the day is cool, and the shadows flee away, 
I will go to the mountain of myrrh, to the hill of frankincense.
You are all beautiful, my love. There is no spot in you.

Song of Solomon, attr. to King Solomon, c. 950 BC

Possibly of Canadian origin, Red River Valley was nonetheless very popular in the US. It was established in Canada by 1896 and may have been composed in the 1870s. It begins "from this valley they say you are going...."

Stephen Foster's Beautiful Dreamer probably dates from 1862.

The Star Spangled Banner begins "o say, can you see...?"

Another answer notes Schubert's Doppelgänger, which sets an untitled poem by Heine. There is also Du bist wie eine Blume, by the same poet, writing in the early 19th century.

Schubert also set both of Goethe's poems with the title Wandrers Nachtlied, written in 1776 and 1780. Both are in the second person, though the first may be addressed to the divine and therefore not anonymous.

Going back a bit farther and returning to English, Shakespeare's song "O Mistress mine" from Twelfth Night, 1601 or 1602, has long been a favorite lyric of composers. "... where are you roaming? O, stay and hear your true love's coming..."

As noted in the other answer, this is a literary device that has been in use for centuries, and it should not be difficult to find examples of its use throughout the history of western lyrical poetry.