Quite a lot of users keep uploading daily. If you do that, your "Streak number" grows. Once you stop uploading for more than two days - the streak number goes back to zero (you can still upload those missed photos though). You can see a list of users sorted by their streak number here: https://tookapic.com/projects?sort=streak
As backend framework we're using Laravel hosted on digital ocean. Images are hosted on Amazon S3, we also queue uploads to S3 with SQS. Imgix is great service and I can't even imagine building tookapic without it. For database we use postgresql. Any other questions :)?
Actually we're testing premium features, like custom domains and themes. username.tookapic.com would be reserved for premium users. Right now pawelkadysz.tookapic.com points to 404 (which should be fixed next week). But the custom domain hooked up to the site works just fine, see for yourself: http://daily.pawelkadysz.com - this is the kind of premium theme I'm talking about (it's all powered by tookapic).
As for the slugs and URLs. At first we had it like this: tookapic.com/username/ID-slug but we had some issues with people changing their usernames (that's allowed) and putting weird characters in the photo titles, which all resulted in breaking the links and pointing to 404 in the end.
That's why we changed it to how it works now. No username change nor emoji in photo title will break the link now.