For average people, relentless promotional self-branding is basically absurd - it is like the twit who would rush to school to see the grades the moment they were posted - yah, you can do this, but most people would say, "get a life."

Speaking as a boomer, I would say most people like Facebook just because it provides for nice social interaction - it lets you keep up with people you want to keep up with in a non-intrusive way.

Twitter is good for specialized uses. If you have an expertise, or a strongly-typed business niche, your followers can keep up with your periodic announcements relating to your area. Since nothing meaningful can normally be communicated in 140-character chunks, tweets often are just pointers to more meaningful content. This is a non-intrusive way of pinging people for such business ends and works better than Facebook for this purpose.

I would guess Gen-Y doesn't yet have as much need for the specialized-use forum as it does for the social aspects of Facebook. Just a guess, though, since I am too far removed in age to know.