My co-worker spends most of her work day on this site, how can I go about telling her to get to work nicely?

She is a nice girl and usually is a good worker but for the past couple weeks she is always on this site and is not accomplishing her duties, I want to tell her nicely to work and I do not want to have to take away internet privledges.
Update: I posted it on here so she can read it, that was my first post, so cut the crap about me being on it, and yes I am the supervisor
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