The reason we were getting closer and closer to being positive on variable costs (looking at revenues on a sales recognition basis, since from the sales volume perspective, things were already positive in the last few months) is, yes, improved monetization, but more importantly AWS optimizations. We had squeezed a lot out of S3, then EC2 and our last step, the one we were working on before shutting down, was RDS where there was a ton of room. Looking at our trajectory, I'm pretty confident we would have been positive even on AWS (but unlikely much). You can even compound that with AWS discounts which apparently even startups can get looking at other comments in this thread.
To re-iterate, you would not build a freemium business like Everpix, with intense computing and storage requirements, at scale on AWS. Large photo platforms have their own storage and servers. Everpix was never intended to grow out of AWS either and you know you can cut your infrastructure costs to at least half. That was the plan post Series A. Most CTO / CEOs of large established photo companies I talked to said they were doing much better than this 1/2X, so that's conservative (I've been told ranges of 3-8X in savings vs S3 for instance).
Anyway, Everpix was bringing about $6 revenue / user / year IIRC, which is the part that really matters considering the denominator, infrastructure costs, is something you know you can really bring down. One relevant reference point is 500px for instance which is doing about $1 revenue / user / year [1]. I would be surprised if they were growing "sustainably" too ;)
[1] http://techcrunch.com/2013/08/07/500px-scores-8-8m-series-a-....