> Tumblr's success was that it was an adult space that wasn't strictly pornography.
This is a common talking point here, but it just isn't historically accurate in terms of absolute numbers.
Tumblr was most successful in 2011-2012. It started getting huge in 2010 in large part due to having built-in photo and video support, at a time when Twitter didn't have these, and Instagram and Pinterest were still both tiny. Tumblr also offered built-in reblog functionality before Twitter had built-in retweeting.
For a little while there, Tumblr was a mainstream social network -- certainly a bit edgier and more liberal than most competitors, but still mainstream enough that many major brands had a Tumblr presence right alongside their Facebook and Twitter icons. Adult content was definitely present, but it was a single-digit percentage of posts at the time of Tumblr's peak historical traffic levels.
A lot of those mainstream users started to decamp to Instagram and Pinterest after that point, depending on their personal use-case.
That adult content percentage definitely rose over time, but timing-wise that coincided with total traffic decreasing, not increasing. It also scared off the brands, whose advertising dollars were sorely needed to achieve sustainability for the business. So to me, attempting to attribute Tumblr's "success" to adult content just seems way off-base.
Source: I was personally responsible for a decent chunk of Tumblr's scalability and capacity planning for several years.