The C10k Problem, WhatsApp and more have over the years shown how web server application have improved to handle a high number of clients simultaneously. High Scalability offer further food for thought:
In 2011 WhatsApp achieved 1 million established tcp sessions on a single machine with memory and cpu to spare. In 2012 that was pushed to over 2 million tcp connections. In 2013 WhatsApp tweeted out: On Dec 31st we had a new record day: 7B msgs inbound, 11B msgs outbound = 18 billion total messages processed in one day!