that's because 'online' doesn't do much selective filtering. Lets say you go the symphony (of the orchestral type, not the framework) and meet some boys. You are selecting, let's say you go a coffee shop in the Mission, or Lower Haight, again you are selecting. Sitting in Dolores park on a sunny sunday afternoon automatically selects certain demographics. When you look for boys or girls online you use primitive criteria for selection which results in poor matching. Facebook is a much better dating site because groups are more organic, formed by real people coming together around common values.
I am rather surprised that Julia has any problems meeting boys, I'm sure she could just sit in a coffee shop in the Mission with a laptop (preferably with a few stickers, including the obligatory CC) and be approached by decent young men all day.