Facebook is one technology firm that managed to go public - list its share on a public stock exchange successfully - extremely successfully in fact.
But other Silicon Valley companies have had tough time selling their shares. Uber and Lyft, the taxi companies went public this year and since then their shares have tanked.
Sarah Lacy, founder and editor-in-chief of the tech news website Pando, explains the difference between Uber and Amazon, which also was a loss-maker in the beginning.
"Everyone criticised that Amazon lost money too, but if you compare the actual companies at their time of IPO, they're very different stories, and Amazon lost money until it dominated the core market, and then used that dominant position in that market, which was profitable, to then expand into other areas and invest in what it's doing," she told the Today programme.
"Uber, on the other hand, is losing more and more money quarter after quarter. Its growth is slowing, which is the big thing you cannot do if you're going to lose this much money. There is no end to this money losing in sight, and it hasn't even dominated its core market."
She adds that Uber has also "utterly failed" in its autonomous cars efforts, citing the fact that the head of Uber's self-driving project is now facing criminal charges over allegedly stealing car technology trade secrets from Google.