Due to the way its system was constructed, AOL knew not just that people were logged on but which users they were.
This allowed for the construction of a location tool that proved extremely popular.
Appelman joined after his time at IBM, where he worked on some of the first standards to connect computers over the Internet (through what are known as Transmission Control Protocol/Internet Protocol or TCP/IP).
Before building a messaging program for the Internet, he created something else that would eventually spawn AIM.
The rise and fall of AOL Instant Messenger rivals them all."They didn't have any central presence information," Appelman said."They didn't know anything about [the users]." Not so for AOL.Once the dominant force in digital messaging and a source of innovations other companies spun off into billions of dollars of businesses, AIM is now mostly dormant.Mashable sat down with three of the early engineers of the program to learn about its origins, why AOL never quite embraced the concept of a free messaging service, getting hacked by Microsoft and the features that never quite made it to users. It had risen above competitors in Prodigy and Compu Serve to become the dominant Internet service provider for American households.In many ways, AIM was right in line with the times, just at a company hanging on to a business model that would soon become obsolete.