Though the product was relatively simple and wasn't compatible with all of the applications it is today. The team behind it were extremely passionate about it and were great at customer development. I'd send a tweet with a product idea or reply to a post on their blog and I'd near-immediately see a reply. As a customer this tells me they're in it for the long haul, they aren't going to pivot next week and it's worth devoting myself to this platform. They are doing all they can to make a great product and share it with the world.
The value of interacting with your customers is an art that they have to an extent mastered. When you tweet or email buffer you quickly receive a reply, in any article about them the team is thanking users. If you build a great product and really engage with users. You'll definitely build a troop of evangelists willing to pay for the product and spread the world to as many as they can and as you continue to share your vision more of the free users will join this group. This is why they are gaining traction, being built into more apps as a sharing option and I wouldn't be surprised if more are paying. The pricing is dead simple, they could easily charge much more.
Yes Buffer, you know how to game the press. Yes, you are hustlers. You have controlled HN better than anyone I have ever seen (the herd mentality on HN is very real, you know this), but man, there is such a thing as over-exposure.
If HN truly is about learning about and helping other startups, Buffer needs to take a back seat and let others get some exposure.
P.S. That is the startup "secret". Publish on multiple blogs, maximum leverage with your email list, make alliances with other apps, guest post on high traffic websites and game HN RELENTLESSLY. Rinse and repeat. Rinse and repeat.