In 2010, volume 57 of One Piece sold a record-breaking 3 million copies in its first edition. That same year, volume 54 of Naruto sold 1.5 million copies and volume 27 of Full Metal Alchemist sold 1 million in their first printings. The above two are merely the outstanding mangas, as most do not even go above 1 million sales.
One Piece sold over 40 million copies total last year, which is roughly 6 percent of all manga sales in Japan. With the One Piece gallery exhibition going on in Tokyo, the series seems to show no sign of slowing down.
In fact, the series is so popular that there are professors who goes so far as to analyze the story and write a book on it. Professor Yasuda of the Kansai University believes that One Piece's uniqueness lies in the characters' strong bonds with each other—that One Piece's Luffy emits a leadership that many real life organizations should look up to.
With other underlying themes, One Piece has been read, viewed, and loved by people of over 35 countries since 1997.
On the other hand, the manga market seems to be going downhill with no signs of stopping. With its peak sales reaching over 300 billion yen in 1995, research has shown that the sales have halved to 160 billion yen last year. Researchers believe that the declining birthrate is responsible for the sagging manga sales. The number of children is lowering, and adults are buying less and less manga.
Because One Piece seems to stay so strong, publishers and editors are now focusing on ways to keep manga entertaining in the long-term rather than creating pieces that people immediately forget about.
ワンピース止まらぬ快進撃 [Yahoo! Japan]