From Losing the Signal, pg 203:
The most compelling feature driving BlackBerry sales in the developing world wasn’t wireless e-mail but another application that had been included with devices since the mid-2000s: BlackBerry Messenger. While BlackBerry was losing the app race in North America, BBM was establishing itself as BlackBerry’s first “killer app” since wireless e-mail, and the first globally successful app of the smartphone era. In late 2008, 4 million people used BBM. Two years later the number reached 28 million, and by September 2011, it was 60 million.
And its most innovative defining characteristic was a simulated sense of presence:
The three struck upon the idea of creating a “data call”: simulating a phone call through a series of instant messages between BlackBerrys. Creating the data equivalent of an interactive voice call would require the “speaker,” or sender, to know his message was both delivered and received. The recipient would have to know that once she opened the message, the sender was informed it had been read. People would no longer send data messages into the void, unsure if they arrived or were read.
As they note on pg 206, the next generation of services succeeded through their capacity to be used across platforms:
BBM’s success was bound to spark competition, and in 2009 and 2010, a rash of rival mobile instant messaging services began appearing around the world, including WhatsApp, Line, WeChat, and KakaoTalk. Unlike BBM, they worked across multiple platforms other than BlackBerrys and drew millions of new users each month. The one that irked RIM the most was Kik Messenger.
But the refusal of RIM to open BBM across platforms meant that their early lead was lost:
Kik was hatched by a University of Waterloo engineering and mechatronics student named Ted Livingston who had completed three co-op work terms at RIM, including time on BBM. In 2009 Livingston decided to forego his fourth year to develop his fledgling start-up. His initial plan was to develop a music-sharing service that worked with BBM, but after several months he decided to instead create a BBM-like chat application that worked on all smartphones. Livingston approached his former superiors in early 2010 and encouraged RIM to broaden BBM to work on non-BlackBerry devices. With its head start in mobile instant messaging, he believed RIM could crush any rivals if BBM was available on all platforms. He even told RIM in March 2010 he would stop developing Kik if BBM went “cross-platform.” But the company wasn’t interested. 14 “They absolutely refused and they said, ‘No, we won’t do that,’” Livingston said in a 2013 interview. 15 “To be fair, from their perspective, it was hard. They said, ‘People are buying BlackBerrys for BBM.’ That was a real risk.”