Since I upgraded to 3.9.0 earlier today all my social and other plugins generate URLs with the www.domain.com part repeating and therefore breaking all URLs

the same is happening with your forum here
www.artio.nethttp://www.artio.net/suppor...plugins/page-2#25884

Can I downgrade to 3.8.3 and if so, where do I get the paid version from?
my domain is www.deepershades.net

I'd really appreciate a swift response ..

Hello,
that is correct. We have detected this problem with some components.
Patch 3.9.1 fixes this - at least for most components, where this problem has appeared. Please, report back in case it would not help in your case.