It looks like Instagram has been using pg_reorg (the ancestor of pg_repack) to keep all likes from the same user contiguous on disk, in order to minimize disk seeks.
This is very similar to what I'm trying to achieve.
The article is 3 years old. I'd be curious to know if they still do that.
If what they did 3 years ago is similar to what you are trying to do today, who cares what they are doing today? (Besides using pg_repack instead of pg_reorg, of course.)
For what it's worth, we use pg_repack on a regular basis and it works exactly as advertised.
