- Internal identity - no anonymity. This means control over communications, which means spam is manageable. The FB equivalent of spam is metastatic "apps", but, for the moment, you can opt out of those. Spam free communication environments are worth much more these days than they were 7 years ago.
- It's AOL 2.0. I remember when AOL was interesting, back when it was a Mac only spinoff of one of Apple's many failed online communities. I'll call that AOL 1.0. Of course in those days there was no spam, no phishing, no viruses -- essentially the proto-Net was risk free. That meant AOL didn't have an enormous amount to offer, but it still did quite well. Now the Net is extremely risky, especially for XP users. AOL 2.0 has a much bigger value proposition than AOL 1.0.
- I love pub/sub, especially as implemented in feeds and readers. Unfortunately, this technology was a bridge too far for the vast majority of humanity. Only the uber-geeks knowingly use feed readers like Google Reader; all the good desktop XP feed readers have died. Facebook is all about pub/sub, but they've made the technology feel natural to their base. That's a real accomplishment.
- Facebook has shown (sigh) that logic and usability are not all that important for a social application.
The dark side of FB, of course, is data lock. (Privacy you say? Surely you've given up on that 20th century dream.) They're providing more APIs and sharing more identity information than they have, but I would never put my photo library on FB. It's a place to put things that are intentionally transient.