Now those critics will have something else to get up-in-arms about: Izea is planning a launch of a new ad platform called "Sponsored Tweets." And just like it sounds, this new spinoff will pay people to tweet.

Murphy believes the community will be self-policing and the sponsored messages won't get out-of-hand since too many paid tweets could easily alienate a Twitter user's followers. And because some campaigns will pay based on the number of clicks a link receives, that's not something the pay-per-tweet participants would want to risk. (Other campaigns will pay a flat rate per tweet).

Izea isn't the only company using Twitter as an ad platform - the pay-per-tweet service Magpie has also been running campaigns for some time. However, on Magpie the companies whose sponsored messages were being tweeted out weren't necessarily the ones buying the ads - it was outside affiliates. Of course, that only came to light after we revealed some of the participating company names in a blog post back in April. Still, the set up there is arguably much more spammy than with Izea who plans to work directly with advertisers.

The only question now is how big will the backlash be this time around? Is this a better or worse idea than paid blog posts? Will you immediately unfollow anyone whose posted a tweet tagged #spon? Or only if they go overboard? Share your thoughts in the comments.

But there's still a small problem here. Most Facebook applications aren't worth paying for. "It's sort of the chicken and the egg scenario," says Zuora chief executive Tien Tzuo, "we're providing the chicken, but are hoping it will hatch an egg."

It's not as if there isn't potential for a paid application marketplace. Facebook hosts some 50,000 apps and there are more than 600,000 developers on the Facebook platform. The problem is that a good bit of these apps are time-wasters, games, trivia, quizzes, and other somewhat non-essential applications - hardly anything worth your hard-earned money.

Z-Commerce, though, can imagine all sorts of applications that could take advantage of this new e-commerce platform. We would love to see professional, useful applications such as they ones they've envisioned - things like LinkedIn Recruiting, Relationship Managers, and other imaginary - but exciting - application ideas.

To get the ball rolling, Zuora is launching a contest for which they're seeking five Facebook developers to be the very first to build a subscription-based application with Z-Commerce for Facebook. Selected developers will receive free access to Z-Commerce for Facebook for one year. For more details, visit http://developer.zuora.com/facebook.