Tech startups have been avoiding the public markets for a few years. Only 19 have gone public in 2017, higher than the number last year but well below average. 

Roku went public a week ago with an offering price of $14 a share, the top end of its proposed $12 a share to $14 a share range. On its first day of trading, the stock popped up 67 percent to $23.50 a share, giving it a market value of $2.23 billion. Since then, the stock has fallen back to close at $20.85 a share yesterday.

Roku is best known for its streaming-TV boxes and sticks that offer access to Hulu, Amazon, Netflix and many, many others – it claims to offer more than 5,000 channels on its platform in the U.S. and 3,000 internationally. In the tech world, it's also known as a small player that has done a remarkable job of holding its own against rival offerings from Apple, Google and Amazon...