Which one is correct?

I posted a message IN or ON Whatsapp Group.

Which proposition, gvien above, is correct to use?

IN or ON

Thanks in advance!

Question

Which one is correct?

I posted a message IN or ON Whatsapp Group.

Which proposition, gvien above, is correct to use?

IN or ON

Thanks in advance!

Answer

You can use either because WhatsApp and the internet in general are new enough that we haven't yet agreed on a formal rule for them. 
In general, we use ON for larger and more public things and IN for smaller and more private things. For example, we ride ON the bus, but we ride IN a car. By that logic, we would post ON WhatsApp (a big public forum) but IN a WhatsApp group (a small private forum). I suspect that is what will someday become the rule.

Answer

You can say either, but you need to remember to use articles (like a/an/the) properly. Moreover, if you want to

For example, you say either of the following  sentences:

(1.)  I posted a message in the WhatsApp group.

(2.)  I posted a message on WhatsApp.

(3.) I sent a message to the members of our Whatsapp group.

Answer

IN would be the most natural-sounding option :)

Answer

"ON"

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