While the FB posts are clear scaremongering, to say that diacetyl is harmless is also false. It is known that diacetyl is harmful to the lungs and I would advise against vaping it. Vaping has only been going on for a few years and so the data right now is inconclusive, although it tends to show it represents a much lower risk than smoking.
Comparing quantities of diacetyl in "e-cig cartridges", which is how they did it in the study that caused this storm in a teacup, most likely means they were not testing custard flavours on an RDA with .15 Ohm coils. The amount of vapur produced by these types of deices is many orders of magnitude greater than some shitty barlites pen.
Vaping itself is not harmless; only breathing clean air is harmless. We should all be mindful of the fact that while vaping is far less harmful to your lungs than smoking, it still isn't good for you: it's just better than smoking. I'm not saying don't vape, I love vaping, but minimising risk by not consuming chemicals known to be harmful surely makes sense.