A tweet from Twitter Inc. announcing its IPO filing is shown in this photo illustration in Toronto, October 3, 2013. Twitter Inc will seek to raise $1 billion in the largest Silicon Valley IPO since Facebook Inc's 2012 coming-out party, according to an IPO filing made public on Thursday. REUTERS/Hyungwon Kang (CANADA - Tags: BUSINESS TELECOMS TPX IMAGES OF THE DAY)