I've still got my nose in facebook, which is proving to be way too addictive. Interestingly, I found Contiki in a related group (though listed as sponsored) on the Explore the World group.
Does advertising belong in social networking applications? I reckon yes, if it's relevant. Contiki as a tour company is clearly offering tours that are of relevance to this particular group. so it makes perfect sense for them to be there. From a marketing perspective, they also using the site to create conversations amongst their potential and existing customers, in the very place where those users are spending a lot of time.
There's also a group for Kumuka travellers, called Kumuka Overland, although this doesn't appear to have been set up by the company - it doesn't confirm this one way or the other. Great for Kumuka if their customers are starting such groups.
The rules of marketing are definititely changing. I reckon travel companies like Contiki, Kumuka and Explore are doing a good job of using new forms of media to get to their customers. Will be interesting to see how things evolve in this space.