WhatsApp has been down all day and RedFoo isn't laughing his ass off one bit ... in fact, the singer says it's totally messing with his business -- so he's taking his patronage elsewhere.
The popular messaging app has been down for a few hours -- just days after being sold to Facebook for 19 BILLION FREAKING DOLLARS -- and RedFoo was pretty pissed about the whole thing.
The LMFAO singer says his girlfriend is also mad ... 'cause he's been MIA without WhatsApp.
The company says they should be up again soon ... but RedFoo says it's too late -- he's already moving to another app.
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