The desk surface/MDF thickness is 5/8". The desktop is not flush with its frame, it hangs over it by a few millimetres. 

If you are interested in attaching monitor mounts to the desk via a clamp system, the thickness of the surface/frame (circled in red below) is 1 5/8".

Please see the manual below. Downloadable/printable PDF version of the manual at the bottom of this article.