Selecting all faces on a plane on imported mesh I am working with an STL file that was generated outside of Blender and then imported. The mesh has a flat plane (the selected face in the photo is along that plane), but it is composed of many faces. My desire is to combine all the faces with the eventual goal of extruding it. In my screenshot I have started by selecting some of the faces manually and combining them. 

I'm looking for a way to select all of the faces on the plane as it is quite tedious if I have to select each one manually. 

Thanks.
Option 1: press shift+g and select normal. This will select all faces with the same orientation.

Option 2: Press some number on the numpad to snap your view to a specific axis. Snap you view so that all of the faces are seemingly 2 dimensional, and you are looking at the from the side. Then you can press z to change to wire frame mode. Next, press b (box select) and drag a box across all the vertices you want.