TrueKnowledge is working to gather and structure all known information so that it can answer any question put to it. This is extremely ambitious and quite hard.
Stackexchange is a spin-off from StackOverflow - a Q&A site for developers that has become enormously popular very quickly (like a year or so). Developers ask a question, other developers answer it, and the community votes up the best answer. Submitting a good answer gets you reputational points. Having more points gives you more voting power. The best answers float to the top, which is why the site is so popular - developers know they are likely to get or find a good answer to their question.
The folks who built StackOverflow (Joel Spolsky and Jeff Attwood) now offer, through StackExchange, their core voting / reputation technology to anyone who wants to build a Q&A site. Like mothers, or business travellers.
TrueKnowledge and Stackexchange are both are aiming to make it easier for you to find an accurate answer to a real-world question. But they are using completely different approaches.
It seems to me that StackExchange sites could well build a useful knowledge base much quicker than TrueKnowledge or Wolfram Alpha could. It is hard enough for TrueKnowledge to answer questions like:
But do people really need answers to "factoid" questions like that? Could TrueKnowledge ever manage to answer real-world questions like:
Even if TrueKnowledge eventually can answer questions like this, I think they will be too late. My money would be on StackExchange sites gathering and organising that knowledge faster than TrueKnowledge and Wolfram Alpha will.