You can be forgiven for occasionally pondering how, with all the skyscrapers, stadiums and buildings on the Vegas strip, the Earth can keep its mass constant. It seems far-out that nothing we do affects Earth's mass. And that's not entirely false: Earth's mass isn't constant but not because we keep on building fancy new arenas for every Olympics. (Anything we build on Earth, after all, comes from existing matter.)
Our planet is both gaining and losing mass all the time. We lose mass due to a couple things: The Earth's core is a huge furnace, and nuclear at that. This means we're constantly losing energy, which means we're losing mass (but probably not more than 16 tons a year) [source: McDonald]. More importantly, light gases like helium and hydrogen are constantly making a break for it, escaping our atmosphere at a combined rate of about 96,600 tons a year! Luckily, we have a long way to go before we run out of our precious hydrogen. (As in, trillions of years).
A factor to offset this mass loss is the nearly 40,000 tons of dust that lands on us every year [source: McDonald]. And not from dust storms -- from space. The Earth's gravity sucks in the bits of the solar system that exploded or didn't form completely into a planet. So, next time you're brushing the dirt off your shoulder, tell your kids that it's just some leftover asteroid.