"Spend" is a very misleading and even distracting metric. Before going to deep into this, it's worth noting that you should never rely on advertising as your sole source of traffic generation when bootstrapping. It gets expensive, and it generally doesn't generate immediate returns. Pay-per-click can be an exception, but that's also an expensive game to get into.

Unfortunately, much of our advertising experience isn't going to be relevant due to how long ago it was. We advertised primarily on FusionAds and Daring Fireball, which has increased from $2,500 to $8,500. We don't have a daily advertising burn, and we never really have as we just dabble in advertising from time-to-time.

I'd be willing to bet that advertising has at best contributed 10-15% to our growth. Given the amount that we've spent, that's a very high cost of acquisition. However, it did help generate some awareness. There's a whole lot of context with advertising. The first $5k was relatively more effective because Sifter was so new. It was great for generating awareness. However, after that initial awareness is generated, unless you're going to spend heavily (more than $10k a month)on a branding campaign any additional impression-based advertising to the same audience generally won't be as effective.