I wouldn't pay for Twitter because I barely use it, but I would gladly pay for Mint.com to get rid of the ads, suck up valuable screen space on an app I use daily. And I do pay for Remember the Milk so I know it stays around. I'd pay for Gmail and Google Calendar.
But everyone is afraid of charging. Some users would surely switch away if you had to pay, but many wouldn't. How much productivity did Gmail originally bring to the table? Did they capture anywhere near that value via ads?
With a free site, the cost of acquisition is lower, you have a big fat user number to quote (even if many of them aren't actively using it, since they don't have to pay, they don't cancel) and the LTV is a future, "undetermined", number. So it's not as apparent that maybe a business isn't generating real value.
Of course, it's also not as apparent if your business is generating real value. If you were looking to make an investment, wouldn't you look for a business with proven value over a business with hypothetical future value?
If there isn't a huge spread between the obvious present value of a business and its /potential/ value, then you can't make an investment where you expect a 4x-5x return. Those kinds of returns only show up when there's a lot of uncertainty around both the current value AND the future value (and most of the value of a new VC business comes from the discounting of its future value, rather than a calculation based on its current performance.)
Twitter is a good example of a product that could certainly use less obtrusive ad placement (i.e. no "dickbar"). However, it's possible to have noticeable advertisements that do not impede the user experience. Google, Bing, and Facebook are all great examples of this.
I realize the article is about more than just advertisements in free software. The common outcomes of free that Ryan speaks of are spot on in my opinion. However, I do feel that free, based on advertising, and good are not mutually exclusive.
I'd like to see it happen, if only for the fact that we'd have a fantastic case study for future generations of social networks.
What they have to be careful of is that if they start charging the non-commercial users, those users will go elsewhere. And if those users go elsewhere, the commercial users will also go elsewhere.
So, if they can figure out how to charge me without charging my customers, they are golden. How to do that? I have no idea.
Of course, that is a gamble (MySpace had the same dreams at one point), can lead to suckier products, and Twitter isn't exactly nailing the execution, but I can see why they don't want to charge.