although there's a relatively healthy number of fixies around here, there's only one semi-regular fixie group ride. doesn't often work with my schedule, but it's fun when it does, since it tends to stay urban and quick without being a race.
otherwise, i mostly ride alone or with my wife (the latter is mostly transportation cycling, but i'm slowly getting her hooked on just riding for the sake of riding). once in a while, i'll ride with 3-4 others, and will join a big group ride about twice a year.
even when i ride centuries (standard and metric) one the fixed, i rarely end up riding with anyone. it seems that if i ride them on the regular roadie, i almost always end up on a pack, small or large, or even a paceline. so maybe riding fixed does somehow tend toward the solitary, but it's hard for me to say, since i prefer to ride alone anyway.
it's been that way since i started riding. when i was racing, i mostly trained alone. when i rode off-road a lot, it was almost always alone, or maybe with 1 or 2 other riders. i always liked time trials best, even though i was never really flexible enough to get into a good TT position and still be comfortable.