During our social media marketing bootcamps and some of the other speaking we do, we briefly cover advertising on the web. The 15-second precis: only spend your money where you can target your audience with razor-like precision, and rapidly measure outcomes. Google AdWords and Facebook ads are two good places to start. It's right around this time that somebody usually raises their hand and offers some variation of "I'm skeptical about your advice, because I never click online ads".
It's a classic misstep of logic that everybody makes occasionally: "I behave a certain way, so my customers will behave the same way". Its assumptions like these that can limit new ideas about reaching your audience.
I also sometimes encounter a certain snobbishness from tech marketers, who assert that most of their geeky target audiences uses browser-based ad-blockers, rendering online advertising ineffective. As with the above assumption, I encourage them to spend a little money and test that hypothesis. We've had clients who get the lion's share of their annual revenue (which was not insubstantial) through judicious use of Google AdWords.
I guess this is just another blog post about assuming, an ass, you and me, but I encounter the behaviour often enough (and sometimes in myself) that it's worth keeping in mind.