Hmm... the terms are the same, but you're right, they're not directly tagged. Also, there appear to be multiple "types" for certain object - like has "bookmark", "like" and "repost".
Although, content type does map these to a class, so could feed into whatever heuristic scheme you're thinking of.
Personally, I'd prefer not to have a db change if it can be avoided.
Possibly the approach should be to refactor the ContentType code into something a little more useful, covering both post types and discovery.