You should read The Atlantic's profile of @HistoryInPics, the Twitter feed that has amassed nearly 977,000 followers in six months by posting cool archival images—copyright and credit be damned. It's run by Xavier Di Petta, 17, and Kyle Cameron, 19.
How do they do it? Once they had one account with some followers, they used it to promote other ones that could capitalize on trends they saw in social-media sharing. "We normally identify trends (or create them haha). We then turn them into a Twitter account," Di Petta said in an IM conversation. "Share them on established pages, and after 50,000 - 100,000 followers they've gained enough momentum to become 'viral' without further promotion."
The trouble is, not every image is copyright-free. The reporter, Alexis Madrigal, is more blunt about it:
The audiences that Di Petta and Cameron have built are created with the work of photographers who they don't pay or even credit. They don't provide sources for the photographs or the captions that accompany them. Sometimes they get stuff wrong and/or post copyrighted photographs.
Their response—seriously, read the story—is cavalier, naive, and raises ethical and business dilemmas about attribution, copyright, and making money.