Bono -- Facebook WON'T Make Me World's Richest Rocker

Looks like Paul McCartney's title as world's richest entertainer is safe ... 'cause Bono says his stake in Facebook will NOT push him into the number one spot.

Bono was in D.C. this morning to attend a fundraiser for President Obama ... when a Washington Times photog asked about reports that the U2 frontman's $90 million investment into Mark Zuckerberg's social network will be worth $1.5 billion by the end of the day ... making him the richest rocker on Earth.

FYI -- It's been widely reported that Bono's investment company, Elevation Partners, bought up 2.3% of the company in 2009.

But Bono didn't appear too stoked about it ... in fact, he cast doubt on the entire story.

That ... or maybe he's just trying to be modest.