Much ado was made about Mark Zuckerberg's vision for Facebook messaging to function as a replacement for e-mail for the next generation. It's immediate. It's simple. It's natural. And of course it's integrated into a website where users already connect with a lot of the people they communicate with on a regular basis.
I was and still am skeptical. I rarely use Facebook messages, and when I do it's often for multi-party messaging. Occasionally I'll engage in one-on-one conversations, and it is handy to be able to quickly share images, links, etc. But Gmail is still my much more comfortable messaging home.
I did notice one psychological difference in the way I use the different services, though. When I send an e-mail, I always include at least a basic signature, "-Ryan", or my full blown contact information signature for work related e-mail. It actually feels a little rude to me not to. This is a little weird, because it's not like the people I communicate with need to see my name at the end of the e-mail to remember who sent it. However, on Facebook I have no such qualms. I rarely ever include my mini-signature.
Is it because it's more like a private forum? Is it because I expect my message to just be part of an ongoing conversation? Perhaps it's the way I relate to the two services based on their names... I would never send a letter without a signature via snail mail, so why would I send e-mail without one. Conversely, I would never add a signature to a text message, so why would I do so on a Facebook message?