Send Shift+Enter to Facebook comment field?
The Google keyboard has a "newline" key (looks like an arrow with a 90 degree angle) that appears only when you enter "shift" mode (eg when you want to key in a capital letter).  In this mode, the emoji smiley face key changes to a newline key.

On some ROMs, you can set an option to change between the emoji key and the newline (aka enter) key, from within the messaging app settings, under "Soft keyboard type".