I gave a brief overview of what to include in the FAQ but this serves as a specific guide on what kind of companies to include.
Any modern startup can be added; aka anything started in the last 6 months or starting up right now.
So long as they have some form of startup funding (or are bootstrapped) and fall into the Web Startup area then they should be included.
Anything fulfilling the same criteria up to 5 years ago will probably count.
If the startup is a huge company now it is possibly still worth adding - however it is also worth trying to put extra information about the growth and investment into the page.
Anything over 5 years should generally not be included - except in special cases. A special case is when you think it is a crucially important or successful startup
(someone might disagree and delete it of course). Companies like Google would count as NOT included; technically a startup by todays standards they are just too big and complex to be worth adding.