3 million taxi journeys in London have been made using the Hailo app

"Three million taxi journeys have been made in the capital using smartphone app Hailo, the London-based tech start-up announced today.
Investor Sir Richard Branson also predicted that the number of cabbies using Hailo — which number 42,000 worldwide, including 13,000 in London — would double over the next year."

Source:  Evening Standard, 23rd September 2013

Earlier - 40% of London black cab drivers use Hailo (Feb 2013)