Based on my personal experience from using both Facebook and Twitter since a few years back, I have come to the conclusion that Facebook and Twitter are completely different creatures. At least, it appears so to me as I use them for completely different purposes. Although other people might use them differently than I do, I don't think I am very unique in my use. Here are some of the main differences.
Facebook is primarily a virtual home for my existing social network. This network existed before Facebook and it would continue to exist without Facebook. I could do without Facebook. Facebook just makes it easier for us to stay in touch. Most of the new people I meet via Facebook (not many at all) are people who know someone I already know, who are friends of friends. I could meet them in real life as well, but it would be a little bit harder. The types of relationships between me and the people in my network are varying and primarily social: family members, class mates, colleagues, friend…the common denominator is me as a person.
Twitter is primarily an information sharing and networking platform. It is a place where I can meet new people and build a personal network that stretches across geography, demography, organizations, culture...Without this kind of platform most of those meetings would never take place. There is virtually no way to discover and meet new people all over the world if I hadn’t access to a platform like Twitter. The primary type of relationship is relationships based on common interests. It is what we share and say to each other that matters.
On Facebook, I keep in touch with people I have met several or many times in real life. Since I trust these people and they know and trust me, the social media that we share with each other and have conversations about does not add very much value in itself. The interaction and conversations with my friends and sense of virtual proximity that they create are more important than the content we interact and have conversations about. This means that the content can be personal, trivial, nostalgic, ironic, sarcastic, provocative, sensitive, excluding...The things we share and say to each other can often easily be misinterpreted and misunderstood in a negative way if moved to other contexts.
On Twitter, I can meet new people which I later on might be fortunate to meet in real life. By interacting and sharing value-adding social media with each other, over time we start to trust each other. Eventually, we might even start collaborating. The social media we share is often relevant, fresh, useful, positive, insightful...The things we share and say to each other can more easily used in other contexts without being misinterpreted and misunderstood in a negative way.
On Facebook, openness is something which is closely related to privacy issues and creates a lot of concerns for the users. Many of the Facebook mechanisms for openness and transparency were not there from the start, but were added later on. Most people expect that what they say on Facebook stay on Facebook - or rather that what they say to their friends stay with their friends. It is hard for a user to understand what is transparent and what isn’t, who can see and access what and how. This creates a lot of confusion and leads to mistakes when people share things they only expect their closest friends would see but which are actually seen by friends of friends, or even by anyone.
On Twitter, openness is the over-arching design paradigm and the basis upon which value is created. Anything I share is accessible to anyone, and it's natural to think about this before sharing. I don’t have to think a lot about what others can see and what they cannot see. This makes Twitter a much easier environment to understand when it comes to sharing and privacy.