I wouldn't even go that far. I've never visited any City forums other than Bluemoon to give a comparison, but in the case of Liverpool and United fans I would say that posters on any particular fan forum offer a reflection (at best) of the views on that forum alone, and not of online forum fans of the club they support as a whole, as you suggest.
For example, I also use a LFC site called The Liverpool Way, which, aside from the fact that it's full of Liverpool fans, is quite different in tone and content than RAWK. Indeed, the cringe levels expressed towards RAWK from TLW are just as prominent as they are in the 'Meltdown' thread on here. And, during the 'great Rafa debate' in 2009/10, RAWK was wholly behind him, whilst TLW had this whole 'Anti-Rafa Taliban' thing going on. Likewise, anyone browsing the Republik of Mancunia site will see that it attracts a different sort of United fan than you'll see on here, and with behaviour and views tolerated that would be discouraged (or banned) on here. Even on Arsenal sites (which I frequent less) I notice that the Anti-Wenger brigade are vocal on Goonersworld in a way that they aren't on Arsenalmania.
Of course these are generalisations, and some fans post on more than one site, but, as in normal life, I think fans of a particular club, offered the option, will naturally gravitate towards a site that provides a platform where their views will be shared rather than mocked.