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Successful viral marketing promos in FarmVille have been quickly passed along to two other Zynga hits: Cafe World and Mafia Wars. Whether they will see similar traffic boosts remains to be seen.

FarmVille bucks the no-growth trend, passing 30 million by twisting the “gifting” mechanism from sending gifts to asking for gifts.

Zynga innovates with a progress bar to get users to sign up for mutliple communication devices. Playfish’s Gangster City doesn’t look like it’s sticky enough as DAUs flatten out. And Zynga Poker touts 6 million daily users – but they still haven’t reached that number yet on Facebook.

Counters are coming as a replacement to the much-abused Notifications API – how will developers use them? Zynga gives us a first clue.

Now that Mafia Wars by Zynga is charging close to $6 for the new Thailand expansion pack, does this new wrinkle in the freemium model for social games signal an inevitable emulation of MMORPGs?