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After 6 months away, early Facebook designer Aaron Sittig is back at the company as "product architect."

It's a big deal.

If it weren't for one very good idea of Sittig's way back when, Facebook might not be the company it is today.

The main reason your grandparents, parents, and all their friends use Facebook is to look at pictures of you.

The reason those photos are so easy to find is that Facebook makes it easy to tag your photos with the names of the people in them.

This simple tool makes it so no one ever has to send an email with 30 photos attached anymore. It makes it so no one has to go to the grocery store and order doubles to be able to mail photos to friends and family.

This feature is the main reason Facebook is such a good photos site.

Being such a good photo site is the main reason Facebook has anything close to a chance of meeting its executives goal to become the world's first trillion dollar company. Today, Facebook users "share" 30 billion pieces of media, including photos, each month.

The reason that feature exists? Aaron Sittig. Photo-tagging was his idea, according to David Kirkpatrick's book on the company's creation, The Facebook Effect.

(Facebook CEO Mark Zuckerberg, by the way, didn't want to add photo-sharing to Facebook at all. But Sittig and early Facebook president Sean Parker changed his mind. Whatever Zuck is paying Sittig probably isn't enough.)

In an email to All Things D, Sittig explained why he's coming back to Facebook:

I left Facebook last June because I’d been there for five years and wanted to see what else I could apply myself to. I didn’t expect at the time that I’d head back.

I spent my time off advising companies and looking for new ideas. But for each idea I wanted to build myself, I kept realizing that Facebook was the only place with the scale and ambition where I could build my ideas successfully.

So that, combined with the steady influx of talented people like Sam Lessin and Justin Shaffer, convinced me to say yes when I was approached with an offer to rejoin.