I claim the ornamental design for a facial sponge, as shown and described.
FIG. 1 is a perspective view of a facial sponge, showing my new design;
FIG. 2 is a front elevational view of the facial sponge of FIG. 1, the rear elevational view, right side elevational view, and left side elevational views being identical to the front elevational view; and,
FIG. 3 is a top plan view of the facial sponge of FIG. 1, the bottom plan view being identical thereto.