Well this is what's written on their website on the home page!
As far as I know(even if I prefer alot more U2 to FR) FR is the major LCCin Europe. Its normal, they have been around for a longer time so have more destinations and thus carry more passengers.
So in what context is U2 Europe's leading LCC? Is it in the context they are the biggest of all LCCs that actually fly to main airports ? (unlike FR) ...
I only see it like that because otherwise I would consider it to be fake publicity because U2 isn't the biggestLCC in Europe !