But I don't think any of these were valued by their rich uncles—they were primarily valued by professional venture capitalists putting money into the companies

But I don't think any of these were valued by their rich uncles—they were primarily valued by professional venture capitalists putting money into the companies at said valuations. Venture capitalists may have flaws in their valuation methodologies, but who's valuation could you trust more than theirs, as they put their money where their mouth is?