Ad-free Social Platform Goes Freemium With Free, Invite-Only Plan, the project that emerged from founder Dalton Caldwell’s desire to build a social platform that wasn’t driven by advertising, is adding its first free option today.

Given Caldwell’s emphasis on creating a product that people are actually willing to pay for, this might seem like a step backwards, or one of those infamous startup “pivots.” However, Caldwell told me that this actually isn’t a change of plans, and to back that up he pointed to his initial blog post announcing the project back in July. The post didn’t explicitly say that that there are going to be both paid and free tiers, but the two positive examples of non-advertising companies that Caldwell cited are Dropbox and particularly GitHub, which both offer free services and then charge for additional features.

As for why he’s launching the free tier now, Caldwell said that he wanted to be careful about growth. That’s important because he learned from his past experience with music service imeem that “startups are kind of like crystals” — namely, “the microdecisions that you make early on” will have a huge effect on their ultimate shape.

In this case, that meant working out a lot of the technical kinks and building out the broader developer ecosystem before trying to attract a big consumer audience. After all, Caldwell said that if he’d offered a free option on day one, “What are they signing up for? There’s no there there.” (Besides, storage is a big part of the division between free and paid plans, so it didn’t make sense to launch the free version until released its File API last month.)

Now, however, Caldwell said there are enough third-party apps that integrate with in some way that it makes sense to bring consumers on more aggressively. (In the app directory, which launched last fall, I count 29 apps for the iPhone, and there are others for iPad, Android, Windows Phone, and Symbian.)

With a free plan, you can follow a maximum of 40 users, you get 500 megabytes of free storage, and you can upload files of up to 10 megabytes. (With a paid account, you can follow an unlimited number of users, store 10 gigabytes, and upload files of 100 megabytes.) If you follow five people and authorize one third-party app, you get an extra 100 megabytes of storage, as does the person who invited you.

And yes, you will need to get an invite from an existing member in order to join for free, at least for now. Caldwell said that’s a way to grow “outward from the core community.” On the other hand, if you are impatient and don’t want to track down an invite, that’s fine — you just have to sign up for a paid plan.