I was thinking about asking a girl to marry me on Facebook messager is it a good idea?

Then you can marry her on Facetime.

terrible

Nope.

I quote Albert Einstein "When technology surpasses human interaction we will have a generation of idiots'.

Your question indicates that "the generation of idiots" is here with us.

No woman mature enough to marry, would be excited over a Facebook proposal, or take it very seriously. It would indicate someone who wanted a reaction from others more than wanting to tell their significant other, in a very special way, that they want to commit to them legally and be with them for the foreseeable future.

Don't use social media to propose. That's for junior high kids playing make-believe. As your girlfriend to her face, if you are both at least mid 20s, have been together seriously 2 yrs or more, financially stable, mature (which sounds questionable), responsible, and ready to legally bind your lives to each other. Otherwise, don't do it at all.

NO it is not OK this is a very personal thing and should only be done face to face

no lol

No. Proposal of marriage should be overall quite formal. It doesn't have to me formal formal, if you know what I mean, but you need to say it to her face to face. Saying it on messenger is literally, way too informal and casual.

no dont be a pussy and propose her in her face

No