If someone really wanted to do this, you would short GOOG or FB and long the nasdaq (QQQ) by a proportional amount. This position insulates you against the market bulls, which could go on pumping the market for several more years. It's a bet that GOOG & FB will underperform the tech sector more widely.

Maintaining a short position also costs money, so this is a further complication. As a retail investor, I don't know if there are any instruments for shorting these big tech co's. I'm guessing that you can do it on interactive brokers (for example) if you post enough margin. DYOR.