still not 100% sure of how it all works, but I think you see the updates of people you're 'following' and they see yours if they're following you, or if you post an update in direct response to them by including @*insert username here* in your post.
I am Samurai7uk if anyone wants to follow me, so to reply to me you'd put @Samurai7uk in your post.
(I think
)