In terms of why people use social media to share historical photos, my experience has been thus:

1)Identity
2)Connection –  with people locally/regionally or to a regional area (place they grew up) – old friends/family or to identify old friends/family
3)Education (Learn about history)
4)Nostalgia
5)Research
6)Organize the world/Understand humanity
7)Art/Creative value (some of just like to look at photos)

There is some research out there on viewing old photographs which support a lot of these elements (especially identity, looking at old photos of ourselves or places we lived helps us define and retain our identity (for those with memory issues).

Articles about looking at old photographs
http://www.smithsonianeducation.org/idealabs/ap/essays/looking.htm
http://www.psychologytoday.com/blog/snow-white-doesnt-live-here-anymore/201112/what-can-old-photographs-and-letters-reveal-about-ou
http://www.psychologytoday.com/blog/unfinished-business/201009/windows-our-psyche