Treasure Isle, Zynga's newest offering to the Facebook masses, displays a phenomenally fast growth rate as over 4 million users have signed up since it went public on April 3.

Indeed, while dedicated stat tracking site pegged Treasure Isle at 4.3 regular million users by Friday, April 9, Facebook's own internal metrics indicate that the number is likely closer to 5.5 million.

Zynga is well known for three things. Firstly, they are by far the most popular Facebook games developer, primarily thanks to the successes of FarmVille as well as Café World, Texas HoldEm Poker, Mafia Wars and FishVille, all ensuring a tight grip at the top of the most used Facebook game rankings.

Secondly, many of Zynga's titles have the uncanny knack of catching emerging fads in Facebook gaming just at the right time, duplicating and then elaborating on other games that are starting to experience decent levels of success.

Mafia Wars was noted for its similarities to Mob Wars (a dispute that headed for the courts), FishVille has almost caught up with Crowdstar's rival Happy Aquarium on the application leaderboards, PetVille outsmarted Playfish's Pet Society, and FarmVille was hot on the heels of Slashkey's Farm Town. Some are already crying foul due to Treasure Isle's resemblance to zSlide's pre-existing Treasure Madness.

Thirdly, part of Zynga's expert timing is no doubt assisted by iyd expertise in leveraging its existing community of players, cross-promoting new titles across existing games, creating demand as much as responding to it, and rewarding fans for recruiting new players.

That has certainly been happening, but at this stage none of the company's other top games appear to be suffering from a mass migration, and Treasure Isle itself is already its eighth most popular title. That peer persuasion element is another ingredient in the company's recipe for success, pulling players back in just as they were about to give up.

If the Isle can even double its population in a week's time, it will already be among Facebook gaming's top 20. Simply by elaborating on a decent gameplay mechanic and giving it mass exposure, those heights are not far off. From there, who knows what will happen?

Top ten fastest growing Facebook games, Friday, April 9

1. Treasure Isle (4,324,406 = +4,324,352 / 8,000,000%)
2. Hotel City (4,280,401 = +3,345,159 / 357.7%)
3. Aquarium Life (2,508,219 = +1,924,984 / 330.1%)
4. Social City (12,174,425 = +1,268,589 / 11.6%)
5. Texas HoldEm Poker (29,989,214 = +818,711 / 2.8%)
6. Zoo Paradise (3,018,766 = +743,526 / 32.7%)
7. Bubble Island (5,119,306 = +721,724 / 16.4%)
8. Ameba Pico (1,500,824 = +599,677 / 66.6%)
9. PetVille (21,415,966 = +560,175 / 2.7%)
10. Family Feud (863,712 = +527,106 / 156.6%)

Numbers in brackets relate to total monthly users, the increase in monthly users, and the weekly percentage growth.