This free app lets users swipe “yes” or “no” on potential matches. If two users swipe “yes” to each other, they are then able to text through Tinder’s interface. Launched in September 2012, Tinder had an estimated 500,000 users in its first sixth months, with a day-over-day growth rate of 5%. Popular with younger folk – it’s average user is 23 – Tinder users have rated each other’s profiles 4.5 billion times, and have been matched up to 50 million times. Tinder works because it takes rejection off the table, and taps into a distinctly millennial activity – texting.