The elastic properties of cubic samples of compressed expanded graphite determined by means of ultrasonic velocity measurements are presented. These materials are highly porous and exhibit porosity-dependent anisotropic moduli. The results are analyzed according to two approaches. The first involves semi-empirical equations fitted to the experimental data, resulting in information about the shape and the connectivity of pores. It is found that pores are oblate ellipsoids, connected parallel to their direction of flatness. The second approach is based on application of the percolation theory near the rigidity threshold. The value of the critical exponent indicates that compressed expanded graphites behave like elastic networks in which central forces are predominant. Results of this study give evidence that ultrasound is a convenient and accurate method for investigation of the critical behavior of the elastic properties in highly tenuous structures.