The term affiliate marketing has taken a bad rap over the years, primarily because people are abusing just how easy this is to do. Internet marketers are finding products they don’t even use because they come with a sweet commission, and are spamming everyone until they either buy, or unsubscribe. This is also known as the dark side of affiliate marketing.
I have not. While I am intrigued with the possibility of making online income, it seems to be less passive then how I want to spend my time. Regarding your blog / site, you have done quite well for yourself. However, you have to keep pumping out content or your site would eventually go out of business. That sounds like more of a commitment then I would want. Regarding your book sales, it is probably relatively passive now, but certainly was not when you were writing the book. Now if you love it, great. Just not for me.
The appeal of these passive income sources is that you can diversify across many small investments, rather than in a handful of large ones. When you invest directly in real estate, you have to commit a lot of capital to individual projects. When you invest in these crowdfunded investments, you can spread your money across many uncorrelated real estate ventures so individual investments don't cause significant issues.
This is the basic mistake they've made: they've fallen prey to the belief that money and meaning are two totally separate things. They've chosen to make their money from something that feels completely meaningless to them (some business they care so little about, they just can't wait to get away from it and minimize their involvement as much as possible), which they hope will buy them the freedom to do something they actually care about.
If you have specialized knowledge in a certain topic, you can put together an online course to teach others. For example, if you have experience in real estate investing, you can create an online course “Real Estate Investing 101”. The benefit of an online course is that once you create the course material, you can sell it to as many people as you want.