Quick test on a text field in chrome on my Galaxy Nexus shows the same behavior listed above: two spaces automatically ends the sentence with a period, but two carriage returns does not.
Re:Tumblr President Steps Down
by burgerbrain on Monday April 30, 2012 @01:36PM
(#3910313)
Presumably if he was adding periods manually, he wouldn't have skipped them at the end of paragraphs.
He could have written this on any mobile computing platform.
That being said, he did use short hand texting "speech" with his "w/".