Try this if Steam won’t start on Ubuntu 16. Will probably be fixed in future releases.
cd $HOME/.steam/ubuntu12_32/steam-runtime/i386/usr/lib/i386-linux-gnu mv libstdc++.so.6 libstdc++.so.6.bak cd $HOME/.steam/ubuntu12_32/steam-runtime/amd64/usr/lib/x86_64-linux-gnu mv libstdc++.so.6 libstdc++.so.6.bak