Sean Parker and Alexandra Lenas Credit: Kevin Mazur/

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

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.

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."

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.