I'm constantly surprised that people actually care about the commenting part; tagging I can sort of see the value in, but only from the perspective that Facebook becomes your main photo library. Granted I'm not the most sentimental kind of chap, but once uploaded photos have been commented on, liked, +1'd etc, how many people really care about that sort of thing? Presumably people have copies of photos they care about or are deeply personal.
Of course, maybe this is the part of me that always goes "Oh yeah, I do that, and that, and..." when I go to seminars on Autism rearing its head. I'm also somewhat bemused by people's film-era photo albums for the most part, so I guess there's that.
s/possessions/data/
16-21 is like a personality black hole for most people. It's a time when you're super self-absorbed and you're living from moment to moment, literally changing every day. If I'm totally alone here then please ignore me, but once in a while I will run into a random picture of me when I was 17 or something and I can't help but think "who the fuck is this person".. and immediately want to bury that particular picture. Pictures with family are a different story, those I treasure immensely. But they are so few and far between, and certainly would not have been considered Facebook-worthy in my teens.
I think it's completely the opposite. It's not bringing your friends to a new network that's the hard part - it's bringing your data (mainly photos). Getting thousands of photos out of Facebook that are nicely tagged and commented on and into another network is impossible. Getting the photos out is possible but it's a lot of work. Switching IM networks is much easier than switching social networks which have so much data we value stored in them.
Another problem is that teenagers are constantly meeting new people. The default is not to ask for a phone number but for an add on Facebook. If you aren't on it you aren't going to be able to keep in touch with and develop a friendship with this new person you've just met.