Have you ever heard of drip marketing? The idea behind it is that you have an asset that people want, often information but it could be anything, and you keep people engaged with you and the product you're marketing by slowly feeding them tiny portions of the full product for free. It's one of the most effective means of creating customer loyalty.
In a related note, the third and final portion of the Portal 2 soundtrack has just been released by Valve. You may now go and pick up the tracks you're missing or download the entire 64-track release in one easy file. Go forth and consume, consumer.
Portal 2 Music [Portal 2 official site]