Advertising Age magazine is out with its annual Top 10 list of the most viral ad campaigns online. On the list, some you might expect like a Samsung ad with Lebron James and that Kony video about child soldiers. 

But consider this: if the makers of Angry Birds had wanted 100 million people to see a TV ad for the game, it would have cost dearly.

“It would be looking at about $20 million,” says Thales Teixeira, who studies viral marketing at Harvard Business School. 

Instead of paying millions, game-maker Rovio made perhaps the dorkiest viral video ever. It shows a scientist on the International Space Station doing some physics demonstrations.

One-hundred-and-nine million views later, that’s enough for the number three spot on this year’s list of Top 10 viral campaigns. 

“When your content is great, your fans actually distribute it for you,” says Michael Learmonth, digital editor at Advertising Age. He says while the top video in 2011 got about 65 million views, this year’s winner — that campaign about Joseph Kony — got over 213 million.

“It just shows you that when the idea is powerful enough, the web works,” Learmonth says.  

Before this year, advertisers might put an ad up online only after it aired on TV. Now, campaigns kick off online. A 90-second web version of a Samsung spot mocking Apple iPhone fanboys racked up nearly 72 million views. 

What makes an ad go viral? Novelty tends to make the public people want to share a video. Marketers have been known to even pay bloggers to generate buzz. Harvard professor Teixera says there’s still one wildcard. 

“Above all, it’s still luck,” says Teixeira.

But marketers are learning fast how to engineer viral marketing success.