I believe the way it works as of right now is as follows. Provided the original wall post is on the first page of the activity page then you can bump it by adding a comment on either the person's wall or on the activity page itself. I am not sure this will bump it on a person's wall or not but once you refresh the activity page it will be bumped. Knowing Vanilla this behavior could change at any moment.
And now they have further expanded on this feature! The moment many of us have long wanted is finally here! Comments on a wall post will now give notifications to the owner of the wall!
I think you can now even bump things that are not on the first activity page. Some stuff anyway. I keep trying to bump that Azure Dragon gif that Shandyr gave me, but it never works.
I've found out that you can bump discussions on the activity page with a post, then delete it (either deliberately or unintentionally) and the discussion will stay bumped and won't drop to where it was.
For an example, see a discussion on Elrandir's wall about the Easter.
The last seen post their is from April, the 6th, and it's still on the first page of the Activity.
I think you can now even bump things that are not on the first activity page. Some stuff anyway. I keep trying to bump that Azure Dragon gif that Shandyr gave me, but it never works.
I've made several attempts. Now is the 9th of April. The discussions from March (I've tried the one from the 7th of March) was successfully bumped. While the discussions from February weren't.
So, it seems you can bump any discussion whose latest post was in the previous month.
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Drowned in notifications
For an example, see a discussion on Elrandir's wall about the Easter.
The last seen post their is from April, the 6th, and it's still on the first page of the Activity.
So, it seems you can bump any discussion whose latest post was in the previous month.