They also had a fair number of female users.
Nothing could have been more enticing. But then, over the years, they've done things to trivialize their community function. They also did a fair amount to infantilize the situation, too. I dealt with it to be involved with new found friends and over time it wore on me. There have been so many things that spawned from this flawed community I can hardly deal with it.
I don't care about the next big concept blog. I don't care about someone's cat or kid. There has been such a dearth of intelligent/interesting blogging I don't even know why they call it blogging. What began with a conversation with my friend Paul about a mutual friend's blog post (about the definition of gyro) made me realize nothing would get better on Tumblr. It's still a functional place but they've limited themselves in a not very smart way.