I love the idea of the third site. Sorry, I don't have a TLDR version of the below.
I don't see it so much like IRC... I see it more like this. When I was listening to Jeff Atwood on Scott Hanselman's excellent Hanselminutes podcast, the conversation went towards the possibility of a third place per stack exchange in addition to Meta Stack Overflow - I think it was referred to as "off-topic".
So I immediately had this flash in my mind, of "personalinjurylawyeroverflow.com" and "surgeonoverflow.com" and "saltwatertank.com" and on and on and on each with their satellite meta sites and their satellite offtopic... this Universe of overflow sites. Escaping the engineering ghetto even. PatchworkQuiltOverFlow.com, and so on.
And in the picture in my head, the "Q&A professional" or "Q&A serious interest" sites were these big motherships, islands to themselves - places to do battle, earn badges, reputation, garner community and build search engine traffic ... but the off-topic questions were something different... they were the social network that connected them - more overlapping.
And I thought to myself "it would be like this if I logged on to FacebookTwitterThingWithStatusUpdatesAndAddAsFriendFeature, but I had my badges and reputations... and all my Stack Overflow buddies were on there and we were all in the same off-topic group, but I could add my brother-in-law over on legaloverflow and his buddy on saltwatertank... and nobody, I mean nobody would ask anyone anything about Farmville ever seriously!" and I think this could work.
If Stack Overflow is the intersection of Digg, Reddit, Wikipedia and whatever, this is the intersection of that and Facebook. But it wouldn't be a place to upload pictures of yourself drunk at the party the night before, it would be what LinkedIn couldn't ever become even if it tried really really hard. Because LinkedIn is just the "off-topic thing" in the middle - they don't have the mother-ships that surround it.
Facebook, LinkedIn, Twitter ... re-imagined. So the lifecycle would go like this.
Bob, a new personal injury lawyer, googles to try to find some answer or other. This might already be implausible, as I have no idea if lawyers google to find answers, but bear with me. He finds an answer on personalinjurylawyer.com. Repeat. Bob notices that this site is typically pretty good at getting him answers.
Being a kind of Jon Skeet of personal injury law, he starts answering questions, and doing a bang up job. He starts to recognize other lawyers, even one of whom is a cute blonde that he went to law school with and wants to date. He wishes he could reach out to her in some way.
Bob notices a link to the "off-topic" site, which the PersonalInjuryoverflow moderators have lovingly named the Legal Eagle, a sort of watering hole place. Bored one Friday evening (hah! lawyers are never bored) he stumbles in and notices that the object of his affection also has an account and is already a regular at the Legal Eagle. He is able to strike up a conversation, wearing his badges and joking about his gigantic reputation he earned on PersonalInjuryoverflow proudly. Much to the delight of Maslow, he asks her out and they are eventually able to pass on their genes.
Bob eventually adds his brother to his friends list, who has an account on SaltWaterTank.com and they use the off-topic megasite to trade YouTube videos of cats doing funny things. He adds his old university buddy who's on stackoverflow.com and they trade drinking stories and talk about the new bands that they're listening to.
I guess I don't see it like IRC... IRC and web chat sites tend to have the almost MMO like the problem that you have to be logged in all the time to get the benefit from the community. I see it more like Facebook, LinkedIn or Twitter, just inherently attached to the overflow sites.
The "main sites" are strictly Q&A - no monkey business. They attract the professionals and people with interests.
The "meta sites" are the meeting rooms - well intentioned moderators and community members speaking in polite tones about how to improve the site.
The "off-topic questions" are the social side of things. Sure, we self organizing Twitter obsessed already blogging geeks have figured it out and don't need it. And sure, Facebook already exists.
But that's no reason not to try to go one up them. :)