Answer
Decreases.
Work Step by Step
When $PbI_{2}$ precipitates, $[Pb^{2+}]$ and $[I^{-}]$ in solution both decrease. But originally, we didn't add any additional amount of $Pb^{2+}$ ions to the initial solution. This means that comparing to the initial saturated solution, $[Pb^{2+}]$ decreases.
