In 2002, Synapse, similar to apps like Pandora and Spotify, caught the attention of Microsoft, which offered him $1 million — they even offered him a job, too. Zuckerberg told Harvard's student magazine, The Crimson, he and Synapse co-creator Adam D'Angelo "wanted to go to college, so we said no."
Ultimately, they decided to make Synapse free to use. Just one year later, Zuckerberg would launch what was then called The Facebook.
Sources: Business Insider, The Crimson