Facebook is up almost 500% from its IPO price, so even the very smart private equity and venture folks can misjudge when an opportunity has been completely milked.
Another reason to go public is because the SEC forces companies with 2000 stock owners (used to be 500 pre-2012) to file public disclosure documents similar to being public, so companies typically go public at that point. I’ve heard that some work around this limit by issuing IOUs indexed to the stock price.
You also can give RSUs, which don't actually become owned by the recipient until the company is acquired or become public. No fancy IOUs required. This IOU is denominated in stock units instead of a cash price.
You’re in agreement with gp- he’s saying that an IPO used to be an opportunity like facebook, but now they just stay private like Uber until they believe most of the value is locked in.