VOIP isn't it. Only the hardcore had third-party VOIP applications like TeamSpeak and Ventrilo, and in-game VOIP is a very recent thing.
Instead, I can break the game's success into three aspects: initial install, short-term retention, and long-term retention.
Getting people to even pick up a game (or register with a webapp) is one of the hardest parts. World of Warcraft came into the space with a huge lead in that respect. With the established base of extremely loyal fans, and giant brand recognition in its space, it was sure to get massive attention. The vast majority of new products cannot hope for a thousandth of the eager launch-day users WoW had. Blizzard reaped the fruits of years of brand-building and marketing.
Short-term retention is covered in other comments. The game runs well on old hardware, looks pretty due to great art direction, plays smoothly, doles out a lot of quick rewards, and so on. Over the first few weeks, the player is bombarded with tons of fun things to do and sharp increases in character power.
Long-term retention is the real magnet, though: the game never becomes a chore until the very end. All the way to the max level, the player is led from locale to locale, challenge to challenge, in a masterly display of game design. Compare the solo-friendly WoW leveling experience with the infamous "hell levels" of Everquest.
And what's more, people seem to keep playing at the maximum level, even if they are NOT tackling the high-end raid content. (For the uninitiated, a "raid" is an extremely difficult, often murderously difficult game area whose enemies can only be defeated with a level of teamwork, personal skill, and above all time investment beyond the capabilities of most players.)
Raid gamers are not where Blizzard is making its bread and butter. Instead, the people WoW is retaining are the people who are doing repetitive, seldom-updated content in exchange for slow, incremental, but guaranteed rewards. For example, there is a Capture the Flag team-battle level that has not changed in years -- and people still play it, over and over, in exchange for a handful of points each time. Thousands and thousands of these points can be traded in for a top-notch weapon.
The final major retention pull for WoW is the social aspect: after a while, you aren't playing so you can have fun. You're playing so OTHER people can have fun. You don't want to let down your team, so you log in for scheduled events whether you like it or not. This afflicts raid and "casual" players alike, to different degrees.
So, you want to make a Facebook app that taps into the same qualities as WoW? Make it extremely fun to use at first; give users a sense of reward and increasing power/ability; then make the rewards come slower and slower, in less and less value, for the same amount of work. Just like the diminishing returns from a drug addiction... oh, and somehow monetize it.
I agree with most of this. EQ's "hell levels" are something of a red herring, however, given your point about the game becoming dull. Hell levels were more of an annoying glitch. Everquest didn't become dull because of hell levels, it became dull because most classes didn't have enough to do in typical combat situations.
Big penalties for failure, a low tolerance for minor mistakes, and a limited set of tools for players to win with meant that typical combat scenarios tended to have very little variance. Players usually did the same thing every time. In WoW there's a lot more room to experiment and recover from mistakes without losing the battle. The result is that for most people, battle tends to be more dynamic.
There's also a huge install base in Internet cafes in China and Korea. College kids spend a huge amount of time socializing online there, which is where the major marketing push is made.
Instead, I can break the game's success into three aspects: initial install, short-term retention, and long-term retention.
Getting people to even pick up a game (or register with a webapp) is one of the hardest parts. World of Warcraft came into the space with a huge lead in that respect. With the established base of extremely loyal fans, and giant brand recognition in its space, it was sure to get massive attention. The vast majority of new products cannot hope for a thousandth of the eager launch-day users WoW had. Blizzard reaped the fruits of years of brand-building and marketing.
Short-term retention is covered in other comments. The game runs well on old hardware, looks pretty due to great art direction, plays smoothly, doles out a lot of quick rewards, and so on. Over the first few weeks, the player is bombarded with tons of fun things to do and sharp increases in character power.
Long-term retention is the real magnet, though: the game never becomes a chore until the very end. All the way to the max level, the player is led from locale to locale, challenge to challenge, in a masterly display of game design. Compare the solo-friendly WoW leveling experience with the infamous "hell levels" of Everquest.
And what's more, people seem to keep playing at the maximum level, even if they are NOT tackling the high-end raid content. (For the uninitiated, a "raid" is an extremely difficult, often murderously difficult game area whose enemies can only be defeated with a level of teamwork, personal skill, and above all time investment beyond the capabilities of most players.)
Raid gamers are not where Blizzard is making its bread and butter. Instead, the people WoW is retaining are the people who are doing repetitive, seldom-updated content in exchange for slow, incremental, but guaranteed rewards. For example, there is a Capture the Flag team-battle level that has not changed in years -- and people still play it, over and over, in exchange for a handful of points each time. Thousands and thousands of these points can be traded in for a top-notch weapon.
The final major retention pull for WoW is the social aspect: after a while, you aren't playing so you can have fun. You're playing so OTHER people can have fun. You don't want to let down your team, so you log in for scheduled events whether you like it or not. This afflicts raid and "casual" players alike, to different degrees.
So, you want to make a Facebook app that taps into the same qualities as WoW? Make it extremely fun to use at first; give users a sense of reward and increasing power/ability; then make the rewards come slower and slower, in less and less value, for the same amount of work. Just like the diminishing returns from a drug addiction... oh, and somehow monetize it.