Avenir in action on superhuman.com

Under the hood

.header-1 { font-family: "Avenir LT Std Book", sans-serif; font-size: 18px; font-style: normal; font-weight: 500; letter-spacing: 1px; line-height: 24px; text-transform: uppercase; color: #FFFFFF; }
.header-2 { font-family: "Avenir LT Std Heavy", sans-serif; font-size: 24px; font-style: normal; font-weight: 500; letter-spacing: 1px; line-height: 30px; text-transform: uppercase; color: #FFFFFF; }
.content-1 { font-family: "Avenir LT Std Book", sans-serif; font-size: 16px; font-style: normal; font-weight: 400; letter-spacing: 0px; line-height: 24px; text-transform: none; color: #E3E7EB; }

Avenir is available from Fonts.com and MyFonts.