I saw a TV programme a few years back about Brasil's top futsal player trying out and playing for one of their top [normal] football sides. I can't remember his name but I think he was described as something like the Pele of futsal. From memory, he was able to play at the top Brasilian 11 man level but wasn't a world beater.
We played futsal in the 70s [it was called indoor football then] as a way to keep fit during the summer. From what i see today, there are a group of people that see it as a chance to represent their country/Province, a lot who play just for fun and a lot of kids that also use it as a way to increase their close ball skills.
I can see why it might be popular in other countries; in places like Italy, Brasil and Argentina, I saw every spare piece of land has a small pitch built on it where they kick the football around [places like under motorway bridges etc] and its easier to get 5 or 6 players together for a team than 11 in some neighbourhoods/communities.