Uber was 12% of total rev at the start of 2016, and 17% later on. WhatsApp was the next largest at 9% for the year. Concentration falls off from there, with the 3rd largest customer at 2%.
Twilio's initial (unamended) S-1 filing uses Uber as a customer case study [1]. They called out WhatsApp as a business risk given that they were and are a variable account (15% of their business at the time). Uber was not, potentially because they're a Base Customer Account, but as noted in in the 10-K [2]: "its usage historically has significantly exceeded the minimum revenue commitment in its contract, and it could significantly reduce its usage of our products without notice or penalty."
[1] https://www.sec.gov/Archives/edgar/data/1447669/000104746916...
[2] https://www.sec.gov/Archives/edgar/data/1447669/000104746917...