However, Vine got acquired by Twitter because it took something that was rather complicated - editing short video - and made it really easy through the use of constraints and simplification. (They were also well connected enough to get an early version in front of Jack Dorsey, and he got excited enough about it to persuade Twitter to buy them immediately.)
If you want to replicate their success, you could start by finding something that people would like to do but is currently too hard, and then by stripping enough things away from it to make it really simple and intuitive. Sounds easier than it is, though.
I wrote a post about this phenomenon here: http://news.ycombinator.com/item?id=5049409. I call it the "magical introduction" in my post, I quote:
"....all the media exposure and growth opportunities that naturally follow all as a result of a magical 'introduction.'"
Sadly, "magical introductions" often get in the way of the best products rising to the top, or even having a fair shot to begin in a lot of cases. Vine is a perfect example where a product was acquired by Twitter before even launching, but as a result when Vine launches it is instantly dubbed the "Instagram of Video" in every news outlet. See the Google News search result for Instagram of Video.
But I don't think "they don't seem to have lots of active users" is true.
Think of them as Harry-Potter-esque animated photos, not videos.
That should answer all of your questions.