I see how Time Trial leaderboards for friends work now
Basically, when you upload a ghost, you get the option of attaching a Miiverse post. If you do, the leaderboards can fetch that post for friends and you appear on their ghost list. But you can upload a ghost without a Miiverse post too, in which case the leaderboard can’t especially go and fetch it for your friends.
Basically,
if you want your times to appear for friends, attach a Miiverse post.
Getting Miiverse posts...
Boom! Friends who submitted a time
and attached a post appear.
Edit: Though it's not perfect. Jucksalbe's time there shows 1:150.215 because that's the best one he attached a Miiverse post to. Downloading his ghost, though, shows a time of 1:47.145, which was his best upload but with no post attached.
Basically, Nintendo, get your act together and implement a proper leaderboard.