You'll notice that social proof is a big theme for Basecamp. Since they've been in business for so long, have so many customers, and especially since they have *notable* customers, they have an incredible asset to draw from. Testimonials from the CEO of Shopify, someone from freaking NASA, I mean come on. The thing I love about the way they use social proof is that they don't skimp out. They've got a lot of it, and they use a lot of it. If you got it, flaunt it.