When is the right time to have my own server? [closed]

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http://serverfault.com – Im really not into server stuff and I have some basic noob questions. I have a site where its databases(MySQL) is getting larger and larger. The site is now hosted on Bluehost. Im fine with that. Questions 1- Is it true that hosting services like godaddy and bluehost is only for small companies? If you have a company like ebay or amazon then you should have your own servers? 2- When is the right time to have an own server? why? Or shall I always stick with a hosting provider even if the site gets very popular(1M unique visitors)? 3- If you can describe it very shortly, what are the advanta (HowTos)