Are you guys aware that it is 'be nice to other ABers day' today?
Presuming we are in the UK and dealing with the common small black ant....
Actually, snaggy is probably correct. I very much doubt whether they'll actually have a nest in the sofa but ants do send out scouts to seek food and when something is found they lay down a pheromone trail back to the nest so all the other ones can follow to the food. Even a few crumbs is a feast to them.
If you see them scurrying in a rough line going back and forth along the same path, this is what is happening.