If by faces, Mr t, you're suggesting people went and looked for a (small) boulder, or stone, in which they 'saw' the face of a departed one, and that stone then came to represent the departed one and was either kept at home (as I think Branwen might be suggesting above) or taken to a communal 'cenotaph' then yes, that would help in a pro vote for the small Silbury boulders possibly representing those who have past away.

'Faces' in the pit-marked sarsens (of all sizes) found in Wiltshire can quite easily be seen if you look at them long enough (there's a smiley face with a stubby nose in the central stone here - http://www.themodernantiquarian.com/img_fullsize/65185.jpg if you tilt your head to the left :-) The absence of such faces however doesn't necessarily mean that stones didn't represent a lost one, or that they weren't placed at Silbury for reasons other than mainly structural ones.