As you discuss in the dropbox link, this is a pretty massive selection bias. I'd suggest that this invalidates any statement made on the basis of these studies about "poly people," since most poly people seem not to be included. People all over the world are poly, in every country, of every race and class.

It's as if we did a study of "rationalists" and only included people on LW, ignoring any scientists, policy makers, or evidence-based medical researchers, simply because they didn't use the term "rationalist."

You state:

While polyamory communities have blossomed for decades in the USA (cf. Munson and Stelboum 1999a; Anderlini-D’Onofrio 2004c), polyamory is still quite unknown in Europe. The social organisation of polyamorous communities is not very advanced in most European countries.

Clearly polyamory is not unknown in Europe, though the word "polyamory" might be. Let's not confuse polyamory, which exists anytime someone openly dates two people, with socially organized communities using the term "polyamory."