Katie Thurston 's sending a message from beyond the virtual grave ... 'cause she's joking that since social media says she's dead, it must be true!

The former Bachelorette shared a clip on Instagram Saturday morning ... thanking randos on Facebook for alerting her friends, family and even herself that she'd passed away after her battle with breast cancer. ABC

She says she's got a message while up in paradise -- chilling with her pal God -- for all of us still holding on tight to the mortal coil ... before plugging a casket company she says won't charge insane marked-up fees. Play video content

It's a pretty hilarious clip ... and, way different from Katie's first reaction to the news -- 'cause she dropped a vid Friday where she called the online rumor "triggering" while talking t

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