New York - Twitter has overtaken Facebook as the most popular social network among US teens, according to a new survey.The study by the financial firm Piper Jaffray found Twitter was preferred by 26% of adolescents while Facebook and its recently acquired unit Instagram were each chosen by 23%.It found that Twitter, Facebook and Instagram remained well ahead of other social networks used by teens. Just four percent of teens preferred Tumblr, unchanged from earlier this year, and three percent named Google+, down from five percent.The news comes as Twitter is readying a $1bn initial public offering, which will be significantly smaller than Facebook's last year but still among the most coveted in the sector. Twitter reports having 218 million active users worldwide."The data point is likely unsurprising as the trend in our survey has been moving toward Twitter over the past couple of years," said analysts Gene Munster and Douglas Clinton in a note to clients."Overall, regardless of the trend in teen preference toward other networks, we continue to believe that Facebook has additional avenues for monetization in Instagram and video in the coming months."The survey of some 8 600 teens showed a shift from a survey earlier this year in which Facebook was preferred over Twitter by 33% to 30% percent.The note that Instagram "continues to look like a smart acquisition for Facebook as it is now tied with Facebook as the second most important social network according to our teen survey".It noted that a plan to include advertising on the Instagram platform would be "a catalyst" for Facebook, for which Piper Jaffray set a target price of $57 a share.