I'm still a bit confused too, but I think it's because I'm American: the terms "language exchange" and "tandem learners" don't mean anything here.
So is it like: say I'm fluent in English and conversational in German; I'd meet up with someone who's fluent in German and conversational in English, and we'd help each other learn? Is that the idea?
So far for example here in San Francisco we have a lot of people willing to learn Mandarin(or actually speaking! I am looking at you Zack! :) ) and fluent in English. Another trend here in the bay is about Korean/Japanese. And a lot of Spanish speakers and Portuguese as well. Those are the main languages so far!
Fortunately, your main page is VERY clear on this main goal. Well done! It makes me wish I were trying to learn a language, as I expect that there's not much use for someone who only speaks English.