Airbnb contacted people who left listings on Craigslist, encouraging them to post it on Airbnb
People who return the most are people following numerous accounts.
So Twitter suggest new accounts to follow
Each email sent from Hotmail displayed the message :
"PS: I love you. Get your free email at hotmail.com"
Automate your manual workflow
Fake a real user behind his web browser
Grab information from others websites
Sending emails, connecting to social networks, ...