With the numbers of sites, I think everything possible should be done in the early stages to encourage users. That means being able to look through & use the site without bothering to register eg. You Tube, to have registration reduced to email / password / username & no forms to complete, & all free.
But once a site is established & is providing major benefits to a large user base, I think it should be able to charge as with any other product or service. Is say $10 or even $20 a year too much to ask of users of Facebook, MySpace, You Tube, Flickr etc ? Compare a year's usage to the cost of a takeaway ? A few drinks ? A bottle of wine ?
Seems to me that the sites went the wrong way in the beginning with the 'free for ever' approach & are now scared to change it.

Interesting point.

If you manage to find a few new features you can offer established users for
a nominal fee, people would likely pay it. One good example would be
Flickr's Pro accounts. I bet if YouTube had used this model, they would have
had the same levels of adoption (the free users stay free, the power users
pay a nominal fee) and they wouldn't be scrapping like they are now.

I think counting on advertising for monetization is a bad strategy. I think free should used to grow awareness and build branding, but a smart strategy would be to know where to draw lines, build walls and start charging admission to the more desirable

I think NOT as long as quality is still associated with these free applications..

Free gets you in the door, and free gets users familiar with a product that they might not otherwise buy outright. I've liked the model of "free" and "pro" models. I'm very reluctant, with the sheer number of new programs and websites online, to simply shell out $10-30 for something that is misleading or doesn't provide what I wanted. For me, it'd be like buying a CD when I've never heard of the artist or their music.

Do you think that some of the companies that have relied on free for so long (take Twitter for example) could reduce a "pro" account without backlash?

I think many companies get too focused on free as a great way to reduce the barrier of entry. But counting on advertising for monetization is a bad strategy. I think free should used to grow awareness and build branding, but a smart strategy would be to know where to draw lines, build walls and start charging admission to the more desirable sections of the product.