It used to be every summer had its song.

No matter where you went, you heard That Song. It spilled from cars, slipped out from underneath ear buds, blared in clubs and floated through stores. And you could never, ever turn on a radio without hearing it.

Last year’s song of the summer was California Gurls by Katy Perry. Remember 2009 when Use Somebody from Kings of Leon was played everywhere? I fully intended to write about candidates for the summer song of 2011, but then I ran into a wall. What, pray tell, determines a “hit song” these days?

In the old days, a hit song was the one you heard on the radio the most. Its radio hit-ness factor was determined by (a) how many copies the single sold; and (b) how many radio stations were playing it. At some point over the summer, one song would be played so much that it became indelibly associated with your memories of that year.

If we’re to use that standard for the summer of 2011, the current leader is Adele’s Rolling in the Deep. No song is being played more often on more radio stations in North America right now.

Today, though, hit songs are determined by more than those two parameters. This is because an increasing number of people now get their music from places other than the radio.

A service called The Ultimate Chart (which monitors everything from plays on YouTube to tweets on Twitter) ranks LMFAO’s Party Rock Anthem as the most popular song of the summer of 2011. At number two, it’s Skyscraper by Demi Lovato, a song that has had barely any radio airplay at all so far. She’s a total online phenom right now. Adele is way down at number eight.

And let’s not forget Rebecca Black’s Friday. As awful as the song is, the video has been viewed 170 million times on YouTube and parodied millions of times more, all without ever troubling even the lowest regions of the Top 40 radio charts. That’s a hit, right?

So what will be the summer hit of 2011? I guess it all depend on where you get your music.