So there you go... must be Facebook inserting it. But why? Maybe related to their "mobile basics" package?
Just checked on a VZW device (normal data plan) in the US and saw the footer on m.facebook.com and it indeed hits a domain like "lm.facebook.com" before redirecting to the Verizon page, which was in German.
I think the message is just something about the domain being inactive. It must be some old marketing WAP page that's no longer in use.