The "Face on Mars" is an example of imagination and wishful thinking. The photographs that show an area vaguely resembling a face on Mars were taken by the Viking 1 Orbiter in 1976. Since then, far better photographs have been taken of Mars (for example, by the Mars Global Surveyor in 1998). They show that the area is heavily eroded, and the "face" was simply a combination of low image resolution and tricks of light and shadow.

Further reading:

Phil Plait’s Web site Bad Astronomy.com. Available at: http://www.badastronomy.com/bad/misc/hoagland/index.html.