They're quite difficult. Some people's first reaction to hearing about them was that stories that compressed would be very hard to write, but I was first intrigued with the idea of writing a whole story in so few words, thinking less words, less time ... yeah, it didn't turn out that way.
I think the hardest part for me is that so much of the story has to be implied, that you have to have a full sense of the character and plot arc before you even start writing; there's very little space to write into and explore the character. Once I get the story, paring it down to the appropriate length I found a little easier, but by then it's already a long way down the road.
This is Twitter "novel" I wrote around Christmas-time 2 years ago, and was published in One Forty Fiction in December of 2011. For me, of the Twitter fictions I've written, it's the one that best conveys a full character and story in this few words: a true "momentary fiction"!
from One Forty Fiction
When donated Christmas boxes came, she gave all the little gifts in hers to the younger orphans. All except the doll. She'd never had one.