Justin Bieber thinks he could have made Anne Frank one less lonely girl ... yes, what you are about to read is shocking.

Bieber visited the Anne Frank House in Amsterdam Saturday. According to the Facebook page for the museum (yes, the Anne Frank House has a Facebook page), Bieber was there with a few friends and bodyguards and went inside while a throng of fans waited outside.

The museum then explained ... "He stayed more than an hour in the museum. In our guestbook he wrote: 'Truly inspiring to be able to come here. Anne was a great girl. Hopefully she would have been a belieber.'"

Had she lived ... the April 13 diary entry would surely have included the word "douchebag."