When random girls add you on facebook, does that mean they're keen? or are they just facebook addicts?

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stupid question I know. I feel like a kid asking this. The reason Im asking it though is because I always seem to get random girls I barely know adding me and commenting on things I post. e.g. A couple of girls from my tutorial class that I have never spoken to (since we only have about 2 or 3 tutorials each semester) do this, as well as a few girls I've seen in my lectures that I have never spoken to (since there are over 300 people in some of my lectures). If random girls do this then are they attracted? or are they just facebook addicts?