Playground Sessions has been in development for more than three years now. The project’s co-founder and CEO Chris Vance says he got the idea by combining popular phenomena from 2009, including not only Guitar Hero and Rosetta Stone, but also American Idol and Khan Academy. “We had this crazy idea: Could we do guitar hero with real instruments?” Vance tells Mashable. via Quincy Jones’ Startup Gamifies Your Piano Lessons.

For a commerce site such as Craigslist, the ratio of buyers to sellers is whats most important. The buyers wont come without items to be sold while the sellers wont come unless theres a market for their goods. If a marketplace was just starting out, it might want to link out to products in a similar marketplace to enhance its own offerings, in order to mimic the look of activity — the single measure which dictates if a social product works. via Why the Web Hasnt Birthed a Prettier Craigslist.