I am having this same issue, if anyone else has come up with a solution.  Specifically, I logged into Facebook, and created an account there, BUT that account is not based on the same email that I used to log into Facebook (done on purpose).

The Facebook version doesn't have a way to log into Plarium to get my Plarium ID, so I don't know how to move it.

It appears that this is a place where people report things to die, because no one has responded here or in Support.

i have found a way you can sign up for a plaryum account on mobile, that account will be linked directly and boom you can play with your mobile account 

Can you explain this better?

What trouble are you having? I don't know if that guy is coming back from 5 months ago, but perhaps I can help.

My facebook ID will not log me onto game, I have it in a screenshot. Support doesnt seem to understand this. Is there any help? or is my account lost?

Maybe you can help me. My FB acct no longer signs me in. I have the FB ID for Mech arena but it does not work. Wished I had known to get a plarium ID

Despite the issues you're having with support understanding, you'll have to work with them. We can't help people directly access accounts they've lost access to here, unfortunately. 

If this is directly a Mech Arena issue you can also try checking out that section of the forum, but support will probably be your best bet. 

yeah any ideas how link fb raid shadow legends with plarium on pc? yet please cant seem do it on mobile.