Some companies make money by licensing software. They are less willing to publish code since, to them, code is revenue and they don't want to give it away. Internet-media companies view code more like a required expense. Giving code away 1. helps reduce carrying costs 2. attracts developers 3. forces out of dependency debt, 4. encourages developers to make their code better 5. builds skills that transfer to other companies 6. make it easier for us to find people who already know the tech we use. etc. etc. There are TONS of upsides. Case in point: we can hire a Hadoop developer. Had we never open sourced the code, we'd have to keep an army of developers in house for a decade. Instead, they can leave to form startup companies (if that's what they want to do) and we still get the benefit of their creative effort. We also invested in one of those companies, so when they make money, we do too. Developers want to work at Yahoo (really) since it will help them build skills they can take elsewhere, or they can stick around and use those skills internally. Either way, why would a developer work on proprietary code when they can work on open source code which will give them more options.
There's a good give and take in the tech world. Sure, our code has helped Facebook and Google eat our lunch. But we don't blame the tech sharing for that -- since they've contributed quite a bit too. We work rather closely with them on a bunch of projects -- which help us all.
Sure, there are some projects that we'd consider the "secret sauce" that really differentiates us from others. We won't open source those. But a lot of code is there 'cuz we need to move bits around quickly. Sharing that code is not going to make or break a multibillion$ enterprise. It's actually going to help make it better in the long run.
To so make money: our sales people to do that, not the tech people. The sales people are given amazing products, and huge audiences to sell to the advertisers. Whereas a podcast might have a million subscribers, a popular radio program have 10 millions listeners, a TV show getting 50 million viewers, or a wireless company have 150 million subscribers, we have over a billion users -- and the advertisers LOVE that. So we all sell ads, and who ever does that better wins. But as tech folks, we're collaborative. It's not really a new thing, it's very much part of the fabric that has helped the internet evolve.