The singer is being celebrated left and right on Twitter right now because on Saturday ... she hit the big 3-1. But more than just sending her well-wishes on her special day -- it seems like people are also congratulating her on this huge purported spike in her net worth.

The number being floated is $800 million. The site CelebrityNetWorth.com is the one that's putting this figure out there, citing the sales of her makeup line Rare Beauty as the X factor.

This new (unofficial) inflated net worth seems linked to a recent Bloomberg report, which says Selena's Rare Beauty blush, on its own, generated around $70 mil last year ... while also claiming RB is expected to fetch at least 3x that amount on this one product in 2023.

If true, it would certainly seem to somewhat back this notion that SG's net worth has, indeed, jumped significantly. Last year, her empire was valued at about $95 million, all told.

It also serves as another testament to the success celebs have found in the cosmetics game. Of course, Kylie Jenner herself became a billionaire with her highly profitable brand -- and now, it would appear Selena is following in her footsteps ... and perhaps even catching up!

Speaking of Rare Beauty, Selena shouted them out explicitly in a birthday post. She writes, "I am thankful for so much in my life and one of the things I am most thankful for is the work we’ve been able to do with the Rare Impact Fund through @Rarebeauty. Because of YOU we’ve been able to raise awareness and increase access to mental health services for young people. This is my true passion in life."

She continues, "People keep asking me what I want for my birthday, and I tell everyone the same thing please do not get me anything but if you want to do something for my birthday, please donate to the Rare Impact Fund."