Assuming you are buying from a reputable source, a silicone bong is perfectly safe. Silicone bongs are safe for the following reasons:
Food Grade-They are made of food-grade silicone, which assures that they are safe for cooking.
Won't Melt-Silicone can withstand a lighter's heat, which means that they will not melt when you smoke out of one.  And if silicone were to somehow melt, it would revert into the safe substances silica, carbon dioxide, and water vapor,