I think the key insight in this article is "A recommendation engine is a feature (not a product)". As a side project I built an e-commerce recommendation engine

I think the key insight in this article is "A recommendation engine is a feature (not a product)". As a side project I built an e-commerce recommendation engine part time over 2-3 months. I didn't want to build out a massive product database and I didn't want to ask users to fill out lengthy profiles so I leveraged existing product and social APIs to handle both. It wasn't very difficult to get something that worked reasonably well - though competing solutions were mostly far worse so maybe I'm selling myself short. In the end it was decently well received (had some press, good user feedback, thousands of initial users) but it failed as a standalone business. Cost of customer acquisition vs customer lifetime value just wasn't there. I've seen no one else succeed in the space so I think the net takeaway is what I referenced at the top - it's really more of a feature than a standalone product. Building the world's best recommendation engine is really just a piece of the puzzle.