Tobold is sure that the raiding population is in decline. I am not sure I agree with his assessment, and he does not really provide any sort of proof that this is the case.
First of all, it's still fairly early in the life of TBC. People are still levelling and working on 5-mans and all the non-raiding content. Raiders are used to devouring content, so it may not seem like a lot, but there's a lot to do.
Secondly, I'm not sure that the actual number of people raiding is decreasing. Judging by the Guild Progression sticky on my Realm forums, I would say that there's at least the same number of people raiding as there were pre-TBC. The only difference is that they are spread across three or four times as many guilds.
However, one thing that is not in doubt is that existing raiding guilds are having troubles recruiting. All raiding guilds suffer from turnover, but there used to be a constant stream of new recruits that would make up for it.
What I think is happening is that people are not leaving their levelling guilds. Pre-TBC, your average levelling guild would maybe get up to 10 level 60s. They wouldn't be able to do anything however. A few would quit the game, a few would reroll alts, and a few would apply to the raiding guilds.
But now, that group of 10 70s will attempt Karazhan. Maybe they won't do very well, or proceed at a much slower pace than the raiding guilds are used to, but it's preferrable to having to leave your friends in the levelling guild.
But the downside of this is that a significant source of recruits for the raiding guilds has dried up. And raiders see people leaving their guilds through natural attrition, and no new players joining, and so complain that there is something wrong with TBC content. That raiding is dying.
But it's possible that raiding is healthy, and may become more popular than ever before.
In any case, what should a raiding guild do for recruitment? My only real thought is to conduct recruitment on a guild level. Allying with a guild that is doing one run of Karazhan could probably net enough people to do the 25-mans.
I think that before we can substantively conclude that "raiding is dying", we need more solid proof. It's possible that raiding is indeed dying, but it is also very possible that the situation I have outlined above is correct, and raiding is healthy. It also accounts for the perception by raiders that raiding is dying.