The following bikes become available when you complete the the corresponding task.
Race 100% wheelie and get 3 stars; do 2 front flips and get 3 stars; do 4 black flips and get 3 stars.
Collect 24 stars in the Desert, Arctic, Dunes; win 3 consecutive races.
Play with 20 Facebook friends.
Collect 12 stars; do a backflip and get 3 stars.
Earn 24 stars each in Desert 2 and Arctic 2; win 12 consecutive races; win 5,000 games in Multiplayer; defeat a Silver Bike in Multiplayer.
High Tech Bike:
Create a game with a Facebook friend, with a random user, via SMS, and via email.
Beat a Police Bike, a Ninja Bike, a Ghost Bike, and a Super Bike in Multiplayer.
Win 250 Multiplayer games.
Play with 5 Facebook friends.
Win 5 games on Multiplayer; Like the Bike Race page; Like the Top Free Games page.
Collect 24 stars each on Hills, Beach, and Savana; win 7 consecutive races; defeat a Bronze Bike in Multiplayer.