Sean Parker's social network just got a little bigger.

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The Napster founder and Facebook billionaire, 33, became a first-time father on Sunday, Jan. 6, when fiancee Alexandra Lenas gave birth to a baby girl, a source confirms to Us Weekly. The couple named their new daughter Winter Victoria Parker.

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Parker announced that his singer-songwriter love was expecting back in September. A rep for the Facebook founding president -- immortalized onscreen by Justin Timberlake in the 2010 Oscar-winning film The Social Network -- told Us at the time that the couple was "very happy and really looking forward to what the new year will bring."

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The tech entrepreneur proposed to Lenas in February 2011. They plan to wed sometime this year.

Parker founded the peer-to-peer music file-sharing service Napster in 1999, when he was just 19 years old. In 2004, he joined forces with Facebook creator Mark Zuckerberg and became the first president of the social-networking site. After stepping down from that position in 2005, he became involved with the Founders Fund, a San Francisco-based venture capital firm. He's currently a backer for the music-streaming service Spotify.