Recommendation sites, like Yelp, have been great, successful experiments in building sustainable social networking communities. Hungry Machine, one of the ETech AppNite presenters, has tied a series of vertical recommendation ideas to the existing social communities, like Facebook. There are a few key differences, however.First, while Hungry Machine uses the APIs of Facebook and Bebo, it is an underlying framework that can allow people to provide recommendations across those communities. Thus, it's not directly tied to any social network in particular.

Second, Hungry Machine works across a variety of vertical topics, namely books, movies, gaming, restaurants, and ... drinking. And yet book recommendations, or the virtual bookshelf, seems to be the most popular. The company claims it gets more recommendations than Amazon. As for Yelp, they are certainly a competitor when it comes to local things like restaurants, the company said. But most of Hungry Machine's applications (like beers and books) don't need a localized aspect.

Another ETech presenter, Tripwise, focuses purely on travel recommendations, and during its presentation managed to throw a bit of a dagger at Hungry Machine, saying that you have to be focused on just one thing in order to be successful.

But many of these applications have been around since last Summer, and Hungry Machine appears to be doing quite well. Next up: linking into Myspace and Orkut APIs.