diff --git a/FAQ.md b/FAQ.md
index 7c63cd4..fd6224b 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -34,4 +34,14 @@ encrypted messages over Tor and store them until the owner collects
 them. It's designed to run on a spare Android device, laptop, or
 Raspberry Pi that's plugged into power and internet. Once that's ready
 it will make an iOS app more feasible. But we are not sure if the 15 minute
-delay will be a deal-breaker.
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+delay will be a deal-breaker.
+
+# When do posts in forums/groups/blogs get shared with others?
+
+A post will be shared with someone if all of these conditions are fullfilled:
+
+1. they're your contact, and
+1. both of you subscribe to the forum/group/blog where the message was posted, and
+1. both of you have chosen to let the other one know that you subscribe (by inviting the other one to join the forum/group/blog, or accepting an invitation from the other one).
+
+When the other person receives the post, they apply the same rules to share it with their own contacts, and so on across the social graph.
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