How can Twitter make money?

This week there has been a flurry of activity surrounding Twitter and their potential plans to charge for commercial use. Then yesterday afternoon the folks at twitter repsponded that there was nothing to report. They basically stated that whatever they do they will NOT be charging.

“Twitter will remain free to use by everyone…”

The fact remains that twitter, although rather well-funded, should eventually make money if they are to remain solvent. The question is how?

I have a completely unoriginal idea. Woohoo!

Google has what seems to be a solid model of ad placement where adverts are visible only to the user and only on their gmail page. Also, these ads are fairly well targeted based on the contents of the email. One note is that the ads are no longer visible when accessing your gmail email by way of an actual email application. I believe few users actually do this as the gmail web experience is very well done.

Twitter, should they decide to place ads needs to do this soon. Their growth is phenomenal right now and the sooner they act, the fewer users will see this as a change. New users will just see this as the standard. My inital thought is that most users access twitter from third-party apps. Jeremiah Owyang shows me that I am apparently wrong.

So, given this, here is the unoriginal idea.
Idea #1: Users would get ads in their feeds. These ads would show up as if someone had tweeted them. They could even be in the form of an twitter coupon. Additionally, users could opt-in to advert channels to customize this information to something potentially more useful to the user. The more activity the people they follow have, the more ads are displayed.

Idea #2: The reverse plan. In this scenario, ads are placed in the stream of your tweets for others to see. They would look like they came from you. This makes more sense for those users who follow/are followed by thousands of people and don’t really listen to the conversation. This would need a great deal more planning on the side of the opt-in selection.

Idea #3: Simply place ads on the twitter page that many users use to access and participate on twitter. This is by far the simplest solution, but as more and more users move to third-party apps, would offer diminishing returns.

These ads MUST provide value to the users and in the end, some combination of all three methods is probably the best way to go. Regardless, some people will be angered by this but not as much as if they actually had to open their wallets to use the service.