When Dalton Caldwell first launched App.net [2] last August, his goal was to reach 10,000 users. Now, 32,000 sign-ups later, Caldwell has begun offering free accounts on his paid, ad-free social networking platform.

Six months after App.net reached its $500,000 funding goal [3] on Kickstarter, Caldwell still speaks of it as if this is all still one grand experiment to him.

"At the time that App.net first had our backing period, there were no mobile apps [4] or desktop apps, and just this awful website we threw together in two weeks," Caldwell says. "This was by no means ready for mass, mainstream adoption. The fact that now there's such a vibrant ecosystem of extremely well-polished apps validates the ecosystem is ready in a way that it wasn't 6 months ago."

The new free accounts are an extension of the 30-day free trials App.net has been testing for the past few months. The new free accounts don't have a time limit, but you do have to be invited by a current App.net member with a yearly account. Free accounts will also have caps on usage, per a typical freemium model, with 500 MB of file storage [5], a 10 MB maximum file upload size, and the ability to follow up to 40 people. (For comparison, paid accounts receive 10 GB of file storage, 100 MB file upload size, and unlimited followers.)

Caldwell, a self-proclaimed Dropbox [6] devotee, has also implemented a referral-based incentive program similar to that company's "Space Race [7]" competition, which let college students compete for up to 25 GB of free storage space for two years. App.net members can earn up to 2 GB of additional storage space by inviting friends. For each friend you invite to the platform, you'll score 100 MB of extra storage, provided that friend follows at least five people and authorizes a third-party app.

Caldwell says he always envisioned App.net moving toward a freemium model, but he says he waited to offer free accounts until now because simply opening up a free tier wouldn't necessarily prove there was a market for a paid social service. He references Dropbox, Evernote [8], and Spotify [9] as examples of successful services that offer both free and paid tiers.

"The ideal freemium service is not annoying, it doesn't nag you, it doesn't blow you up with ads," he says. "It's really porous to get in the door, but it's when you decide that you want to integrate it more into your workflow that you're prompted to pay."

Of course, there are potential downsides to the freemium model--the "Why buy the cow when you can get the milk for free?" adage is one of the reasons why Caldwell wanted to wait until App.net's third-party developer [10] ecosystem had legs of its own. Now, with more than 100 apps created using the App.net API, Caldwell sees App.net in a better position than ever before to appeal to a mainstream audience.

"I don't believe anyone is able to quit their day job yet and just build App.net apps, but I also don't think we're too far away from that, which would be pretty cool," he says.

[Image: Flickr user Dave Hoefler [11]]