* Gaga tops Forbes top 100 list with social media power
* Winfrey drops to second place, earns $290 million
* 17-year-old Justin Bieber debuts at No. 3
NEW YORK, May 18 Flamboyant pop singer Lady Gaga ousted U.S. media mogul Oprah Winfrey to top this year's Forbes celebrity power list released on Wednesday.
Gaga, whose real name is Stefani Germanotta, moved up four positions from last year to claim the top spot in Forbes' annual "The Celebrity 100" 2011 list that measures power by entertainment-related earnings, media visibility and social media popularity.
Forbes rated the American singer-songwriter No. 1, estimating her earnings in the past 12 months as $90 million, but also noting her 32 million Facebook fans and 10 million Twitter followers.
"Lady Gaga beat Oprah this year because of her social media power. She can use Twitter and Facebook to work the "little monsters" into a frenzy that leads to record sales and media attention. She's the best example of how celebrities will need to manage their careers in the coming years," Forbes editor Dorothy Pomerantz said.
Winfrey, who next week ends "The Oprah Winfrey Show" syndicated TV chat show after 25 years, dropped one place from last year to No. 2 earning an estimated $290 million.
Canadian pop star Justin Bieber, 17, was the youngest to make the list and debuted at No. 3, earning $53 million from concerts and sales from his concert movie and documentary "Never Say Never."
Coming in at No. 4 was U2, earning $195 million and Elton John, who earned $100 million.
Forbes said its earnings figures consist of pretax income between May 1, 2010, and May 1, 2011
Rounding out the top ten for 2011 was golfer Tiger Woods, earning $75 million, country singer Taylor Swift, rock band Bon Jovi, British music producer and "The X Factor" judge Simon Cowell and U.S. basketball player LeBron James.
Other big newcomers include singer Katy Perry at No 12, British director and producer Christopher Nolan at No. 47, actress Natalie Portman at No. 75.
Former "Real Housewives of New York" star Bethenny Frankel, who used her latest TV show, "Bethenny Ever After" to promote her Skinnygirl brand of drinks she sold for $100 million, landed at No. 42.
The biggest gainer from last year's list was Leonardo DiCaprio, moving up 56 spots to a ranking of 71.
Dropping out of this year's list that includes film and television actors, TV personalities, models, athletes, authors and musicians, was Britney Spears, who didn't tour in 2010, as well as "Harry Potter" star Daniel Radcliffe.
The collective total earnings of the top 100 dropped this year from last year, from a collective $4.7 billion to $4.5 billion.
Donald Trump, who this week dropped out of a U.S. 2012 Republican presidential bid, benefited from the increased media attention by breaking into the top 20 at No. 17, according to Forbes.
The full list can be viewed at www.forbes.com/celebs.
(Reporting by Christine Kearney, editing by Jill Serjeant)