One day, tumblr user thatsmoderatelyraven (aka Peyton) made a post and claimed that if it got over 500,000 notes her mother would buy her a fluffy chicken. After a bizarre series of events where the circulated post had the chicken's picture continuously changed to completely different images, on February 17, 2013, it hit (and surpassed!) 500k notes and Peyton confirmed that yes, her mom is getting her a fluffy chicken. That date was subsequently declared Fluffy Chicken Day.
There's no way to accurately measure the number of suicides Tumblr has prevented simply through signal boosting and use of available resources, but even one is a CMOA all on its own.
In July 2014, /b/ staged a mass raid on the social justice and fandom sections of Tumblr (which is most of it) in retaliation for a raid Tumblr planned, posting gore, flashing images, screamers and the like in the tags, as well as threatening to make pornography out of peoples' selfies. While the raid is still ongoing, the response from every corner of Tumblr that's been affected has been huge, with bloggers teaming up to warn each other, spread the URLs of /b/'s spam blogs so they can be reported, counter-spamming to drown out the 4chan spam, and generally coming together to prevent people from being hurt by it.