Very good arguments! Twitter certainly has too many mult-level/affiliate marketers and spammers. I've noticed that on Facebook, too, I'm afraid. But on Twitter they can simply start following you. That's probably why they're more noticable. On Facebook connection has to be a two-way agreement.

On the Oprah point: Oprah was a very recent event. Not sure it's had a long enough time to make an impact. And adoption of Twitter has gone up significantly since that spot.