A manufacturer of lightbulbs wants to produce bulbs that last about 700 hours but, of course, some bulbs burn out faster than others. Let $F(t)$ be the fraction of the company's bulbs that burn out before $t$ hours, so $F(t)$ always lies between 0 and 1. Let $r(t)=F'(t).$ What is the value of $\displaystyle \int_{0}^{\,\infty} {r(t)}\, dt$?

$\displaystyle \int_{0}^{\,\infty} {r(t)}\, dt =$