On the one hand it's free work benefiting the corporation... but on the other hand it's genuinely helping sometimes thousands of other people. I benefit massively from reading Amazon reviews, and it feels good to give back. But it is also a contribution that further entrenches Amazon (or Google Maps), it's not like Wikipedia where contributions can be used by anyone.
What do we do when we can help other users, but doing so also supports corporations for free? Although then again, I've never paid a dime for Google Maps and use it daily, it's literally a major part of my life -- so does getting the product for free also play a role?