I wonder if the kind of online environment where the interaction takes place affects the portability and thinness of the relationship. I have a couple of people I've known for nearly a decade through multiple MMOs, and I wouldn't hesitate to have them over for dinner were they in the same geographic area. I've watched them go from single to married to parenthood; from student to first job, etc.

However, for me, Twitter has been more useful for keeping up with real-life friends and contacts. I gave up on Facebook nearly a year ago due to too much noise and not enough signal.