"Join Mobilizon, a Mobilizon instance" : confusing description for most people
"Join Mobilizon, a Mobilizon instance. Mobilizon.fr is Framasoft's instance" : people who aren't familiar with instances won't understand that, imho.
(isssue already mentioned on Framacolibri by someone sometime ago, but couldn't find it again).
I suggest Join Mobilizon on Framasoft's instance, "Mobilizon.fr"