I don't think the author meant that its simplicity (from a user experience perspective) was the reason -- it was what you can do with Facebook that may make it

I don't think the author meant that its simplicity (from a user experience perspective) was the reason -- it was what you can do with Facebook that may make it for people who require less of a cognitive challenge. The best software is so simple that anyone can use it, but the converse (simple software is only used by simple people) is definitely not true.