Nothing Says Stickiness...
...like social interaction.
When i first saw the Internet in '92 or so, while studying at Eastern Arizona College, there was only text: telnet, archie, etc., that HTTP thing reached us much later.
What hooked me (in the net addiction sense) were the social games. Humans NEED social interaction, and the MUSHes and MUDs of the era fulfilled that need for hundreds of thousands of people.
I settled into two environments. The first, a MUSH, featured LISP-like programming tools for users, allowing them to create rooms, bots, and other objects. Users can (fresh code still exists, and there is are active MUSH community) create rooms, objects, robots and AI's to impress and entertain their friends..
The second enviroment was based on mudOS, which provided C-like commands intended for administrators only, players did not create. Even though the framework was adventure-type text gaming, the attraction was social interation.
Most of these were hosted at major universities, and, indeed some of the best MUD/MUSH game programmers threw it all away to become heart surgeons and suchlike.
The problem (IMHO) with these codes, was a license that prohibited commercial use, essentially preventing the kind of cash flow that enables growth.