Twitter dominated by narcissists
In a new study that's sure to give Twitter critics more ammo, Rutgers University researchers reveal that the majority of Twitter users remain self-absorbed tweeters. The study divided 350 Twitter users into two camps: informers (who post informative tweets) and meformers (who post mainly personal updates). Eighty percent of us fall into the latter camp.
While some might interpret these findings as a setback for using Twitter as marketing and community-building mechanism, it's not really. Self-indulgence can be a powerful motivator. It's all in how you leverage that tendency to shape your campaign.

These results may be reflective of our American society at large – if most people are narcissists, self-indulgence may work well as a motivator.
It seems like if you looked at the 20% "informers" separately, nearly 100% of them would be tweeting as a social media marketing strategy.
I think you're right on most counts, except the part about the 80% of meinformers in it just for fun. Of the meformers I follow, a significant percentage are business people who occassionally sprinkle in "meinforms" for business reasons...e.g. "heading to XYC conference to give a preso on social gaming trends. Look me up if U R attending" or "Just published this ebook on Twitter trends. Download it here and let me know what you think" ...that kind of stuff. I also believe there's a lot of blended gray area between the two groups. A lot of meformers inform via Twitter occasionally.