I think the topic of what makes a game "addicting" is extremely interesting. It is an important aspect of game design, but one that some professional game designers seem to be completely unaware of. Replayability is just as important in some games, especially competitive games, as any other aspect of the game. A mediocre game can be made an incredible classic by virtue of its replayability alone.
Zynga is a company that gets it. I can't say I am terribly familiar with many Zynga games, but from what little I know, they stick very closely to the tenet that I am about to reveal. Addicting games offer something very, very simple: perpetual progress.
I am not going to suggest that Progress Wars is the game to end all games, and that anyone who begins playing it is going to have a sudden dearth of will to ever play another game again. I'm just going to use it as the most bare-bones example of my core point. You will gain a level in Progress Wars. You will be tempted to gain another level in Progress Wars. But it's nothing more than a crappy button. That's the whole site. You click a button, and you gain levels. You're not going to spend more than a few minutes playing it, but if you're like me, clicking the close button on that tab is one of the hardest things you're going to do today. You don't want to lose your progress. You want to keep going. It doesn't matter if you already know what's next, you just want to get there so you can feel like you've accomplished something.
The most addicting games really just come down to being clones of Progress Wars. They add tons of bells and whistles (and gameplay, I guess) so that you don't realize that you're really just playing Progress Wars. It's all just smoke and mirrors sugar-coating the same core concept. It's not a bad thing; I get sucked into these games all the time, even knowing all about this. In fact, I specifically seek out these kinds of games, because they are fun.
Here, I want to bring up a game that I consider to be a masterpiece of multiplayer gaming design: Modern Warfare 2.
Every single positive action you take in a multiplayer game of Modern Warfare 2 gets you points. You get points for killing someone, points for capturing an objective, bonus points for kill streaks and multikills and special kills, etc. You are constantly getting points in that game. But beyond Progress Wars-style points just for the sake of points, Modern Warfare 2 uses the points as a means to a broader incentive. Your points dictate your level, and your level dictates what items you can use in game. If you want a better gun, you better keep playing and accruing points so that you can unlock it.
But the game doesn't even stop there. Your gun comes bare-bones. You don't want a bare-bones gun, do you? You want to dual-wield, or have extended magazines, or have a better scope, or make your bullets go through the damn wall and kill enemies who can't even see you. If you want that, you need to use the gun. You get gun-specific points for performing actions with the gun that unlock more and more possible customizations for your gun. They even add aesthetic customizations that require a ridiculous number of difficult-to-get points, just so you have a really long-term goal to strive for.
You get to define perks, such as infinite sprinting, faster reloading, etc. These, too, have points associated with them. If you use your perk enough, you unlock a super version of the perk that does something even better than the original version.
You get titles and icons for performing specific actions within the game. Some of them come with perks or guns or levels, but others come with difficult and impressive feats. You can show off your titles and your level and your guns and your add-ons to the people you play with. There is always something new to unlock, and something new to strive for. And just when you get it all, you have the option to start over. You can un-unlock everything in the game and start clean from level 1 with only the basic items.
And people do that all the time. Unlocking things is fun. It's way more fun than having things. You always want something new, and something better than what you have. And, as Modern Warfare 2 proves, people are willing to give away everything they have already earned just for the shot of earning it again. It's like crack.
Even if you're getting crushed in a game of Modern Warfare 2, you can get a few kills, which gets a few hundred points, and you're that much closer unlocking your BFG of choice. You get rewarded even when you are losing, and that keeps people playing for a very, very long time.
This is the future of multiplayer gaming. This is the reason that I could not get into competitive Starcraft 2. It is not enough any more to merely be a compelling strategy game. Playing is not worth the risk of time wasted if you lose. I want to feel like I have accomplished something no matter what the outcome. I don't want to spend up to an hour of my time playing a game where I lose and that's the end and I have nothing to show for it, and neither does the majority of gamers.
If you are always making progress, there is always a reason to play.