I definitely empathize with your position on this, and you're right that community is the valuable part, hence the focus around active users.
What makes TikTok unique beyond growing like wildfire: their monetization strategy will be easier (the social aspect leads to relevant advertising), and their community is more stable!
The stable community is driven in part by their algorithms, which replace Instagram and Facebook's idea of a feed of those you follow. It gives more power to TikTok, in that you won't notice if some members of the community leave, and they can continue showing you content you'll enjoy from millions of other users.
To run through the list of companies you mentioned:
(1) Snapchat has a solid userbase that's likely to stay with it, and they're worth over $30b. They aren't profitable, but they chose that path when they focused on Spectacles above monetization, and paid engineers more than Google. A $50b valuation wouldn't be unheard of if they had 2x time-on-app as TikTok does.
(2) Vine had hundreds of millions of active users when it was shut down by Twitter.
(3) Tumblr is unique; they cut their own head off when they started removing content. They were always a niche service, without the mainstream appeal and without the ability to target their users with relevant advertising.