These days having few friends frequently signals maturity or coolness - someone who doesn't add everyone they've ever met to look like they have lots of friends.

I think the sweet spot is between 10 and 200 - go over that and people tend to imagine 'there's no way he could actually have that many friends, he just adds people at random and cares too much about popularity'.

Edit: Having said that, I just went back to my fb, which I no longer really use, and I'm on over 350. But largely that's because I've had it for a long time and not removed people I no longer see or have any real intention of seeing, so I don't only have actual friends as friends either.