A long time ago I asked a Brazilian friend, who used to spend hours and hours online at a time, which preposition to use for websites and online services (now apps), and she said "on". I took her at face value, and she was right.

My favorite dictionary online thefreedictionary.com (which features content from the American Heritage Dictionary - my favorite printed dictionary) has a very detailed entry for the preposition "on".  http://www.thefreedictionary.com/on