I had a quick look at them and the feeds looked identical to me?
Most Helpful Girl
My Feed is a tool that displays activity you Follow in real time. On My Feed, you can see all activities from members you Follow, Questions you Follow, Takes you Follow, and Topics you Follow.
Site Feed is a tool that displays members' activity in real time. On Site Feed, you can see all members’ activities.
The Popular Feed shows posts that are getting a lot of attention.
There's more information on these Feeds in the FAQ:
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