The glamorous pop singer had already overtaken US President Barack Obama on the social networking site.
Lady Gaga’s fans numbered 9,940,462, compared to Mr Obama’s 9,444,942 on June 25, the British social-network statistics website measured.
Lady Gaga picture
The 24-year-old Poker Face singer topped 10m on July 2 and thanked her fans on her page.
“I love you guys so much,” she said in a video. “Thank you so much for all of your love and your support, and for sticking by me,” she added.
The group, which gauges online popularity, said Lady Gaga was the most popular living musician online according to her following on Facebook, Twitter and YouTube.
The singer gained more than 1.9m fans alone last month and averages approximately 140,000 new fans each day.
She is ahead of other living musicians such as Britney Spears (3.1 million) and Taylor Swift (5.8 million).
However, Lady Gaga is still behind Michael Jackson, the late King of Pop who died last year. He has almost 15m fans.
Lady Gaga – Most Number of Facebook Fans Record 2010 Video