Google went public when it did because it was effectively already public.
I don't think that's true. Do you have some source that confirms this?
If you go e.g. to Cargill's website (a $100 billion private company), you won't find any detailed financial information. There are some high level numbers and their yearly report is some marketing fluff. Same thing for e.g. Ikea.
As a public company there is relentless pressure to deliver ever increasing quarterly profits above anything else. Trying to focus on anything else (e.g. sustainability or long term plans), opens you up to criticism and lawsuits from your stock holders.
This is such a tired meme. Public companies do all kinds of long term planning.
I love the fact that Silicon Valley thinks there's nothing better for the company than a young, maverick CEO, and that an experienced board and accountability are useless.
Regardless, the 500 shareholder thing specifically applies to American companies as far as I know. [2]
[1] http://www.ikea.com/ms/en_US/pdf/yearly_summary/IKEA_Group_Y...
[2] http://www.investopedia.com/terms/5/500-shareholder-threshol...