A third of tech companies that have had IPOs in the last decade are now trading beneath their initial valuation, according to analytics firm Geckoboard. The underperforming companies include Twitter, which was worth $24 billion at the end of its first day on the stock market and is now worth half that as it keeps looking for a buyer. Other “notorious flame-outs” include mobile game company Zynga, which has lost 66% off its IPO value after floating at $6.6 billion in 2011.