The uncanny feeling upon seeing someone's photo on facebook – someone I don't know, that I feel like I do know.

Freuds theory of the uncanny is related to repetition. It comes from the german word 'unheimlich' which relates to both 'home' and 'unhomely', it is that which we know, that is made unfamiliar (through repression).

In this case, it is that which we don't know 'not on our friend list', made familiar through the repetition of other peoples friend lists. That is, this person is on a lot of our friends' friend lists.

Freud specifically relates an aspect of the Uncanny derived from German etymology. By contrasting the German adjective unheimlich with its base word heimlich ("concealed, hidden, in secret"), he proposes that social taboo often yields an aura not only of pious reverence but even moreso of horror and even disgust, as the taboo state of an item gives rise to the commonplace assumption that that which is hidden from public eye (cf. the eye or sight metaphor) must be a dangerous threat and even an abomination - especially if the concealed item is obviously or presumingly sexual in nature. Basically, the Uncanny is what unconsciously reminds us of our own Id, our forbidden and thus repressed impulses perceived as a threatening force by our super-ego ridden with oedipal guilt as it fears symbolic castration by punishment for deviating from societal norms. Thus, the items and individuals that we project our own repressed impulses upon become a most uncanny threat to us, uncanny monsters and freaks akin to fairy-tale folk-devils, and subsequently often become scapegoats we blame for all sorts of perceived miseries, calamities, and maladies.