Last week I was speaking with a ClickZ reporter about the potential Twitter-Foursquare partnership for an article she was writing. At the time, she didn’t have an angle for the article, so we were simply discussing what it means for marketers and how it might change the industry. She asked me if I thought the partnership would result in the usurping of Facebook’s power and I immediately said “no.”

At first, it was simply because I don’t think Twitter or Foursquare, even with their powers combined, has the user base to kill Facebook. But upon discussing further, we came to the conclusion that it’s because they’re very different channels used for very different things--Twitter and Foursquare are typically used for discovery and exploration. People use them to find new content to read or new places to visit. Facebook, on the other hand, is about staying connected with people you already know, and sharing photos, videos and updates about your life. Sure, the inverse can be true on any of the networks, but it’s not the typical use case.

This difference is why it makes sense for Twitter and Foursquare to join forces. It’s also why Facebook’s purchase of Instagram was so fitting. These pairs of social networks have the same purpose, the same philosophy. And users will maintain separate profiles on each because they offer separate experiences and provide different value.

This got me thinking about why and when we use social media, and how we decide between various channels at any given moment. Not every social media network easily fits into one of these buckets, but I thought it would be interesting to breakdown the main purpose (as I see it) of some of the largest social networks to help marketers understand what role they can play on each channel:

LinkedIn is an interesting one. As hard as they try to be an exploration engine with Pulse and their influencers program, users just don’t want or need it to be that. Their connection engine is strong, the strongest for professional relationship building, and they should stick to and build on their strengths.

What do you use each social network for? Join the conversation on Interstellar's Facebook, Twitter and Google+ pages.