Recently I have found them to be inaccurate at times. I have had neighbors visit my town and Origin says they have not logged in in months.
They are notoriously inaccurate. The best way to see which friends are actually playing is to check the activity level in their towns. this is especially easy during an update, when you can see how much special currency they've collected. (If we get a few days in and I see a lot of zeros, I'll likely delete those people)
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