Is it still the right time to buy Apple, Google and Facebook?

We will use stock's PE ratio to answer these questions. It is an indicator to help investors measure the stock price.
By comparing the stock's current PE with its historical average PE, we can simply see whether the current share price is high or low.
Generally, a low PE might indicate that the current stock price is low relative to earnings.

PE is not the only way to valuate stocks, but past average PE can work as a benchmark when comparing with current PE. This will help us get an idea on whether the stock is 'cheap or expensive'.
And PE is a long-term indicator, so it cannot provide much help for short-term transactions.