Charging for ad-free service never works -- advertisers want to target the very people that would pay for to such a service. If you're an advertiser, you want to target people who are willing to pay extra for a convenience -- the exact people who would opt out.
My favorite example of the author's ignorance: #9. Can you imagine what would happen if Twitter sold its email list? Many of the Twitter users that I know are skeptical about the service, and it looks like the users in general are technically savvy -- all it would take is a really bad PR piece to start a mass exodus.
And that might not be a bad idea. They're best option is to get bought quickly by a social network: They can keep their current service relatively unchanged and bring value to the network through more eyeballs eyeballing more often, thereby raising the owner's ad revenue while not driving away the user base.
If they do something to decrease the userbase in an effort to drive a short term profit, their viability as an acquisition target goes right down the drain.
My suggestion: terms of service that permit free individual use, but require some form of subscription for corporate use.