We assumed that if you are interested in an ongoing conversation you'll either follow it, or by posting in it, you'll be automatically notified.
Having read this, it occurred to me that I'm not "following" individual conversations for the most part, and I wondered why. It might be caused by a simple UI issue:
I'm currently reading your post, which is #2 in a conversation consisting of six posts. If your post convinced me of the conversation what do I do then? I can either scroll up to find the "Follow" button at the top of the page - or skip the remaining posts to find the "Follow" button at the bottom. Either way, I'm going to lose my current position in an ongoing thread. Alternatively, I could wait until I've reached the end of the conversation, and click "Follow" then. This is somewhat realistic in a short conversation like this, but less so in a longer one (where I either forget what to do, or don't have the time to read everything in one go). Overall, chances are that I'm not going to specifically follow a conversation even if there have been interesting posts.
Regarding @JazliAziz' original point of "post bumps", the good thing about them is that they can bring conversations to our attention that we're not following yet, perhaps because we haven't seen them before. This is similar to the "Active" feed, but I feel there still might be a difference to old-school forums in that you saw reverse-chronologically sorted threads per section (~Cake topic) there, while Cake offers a one list for all your followed topics instead.
At the same time, feeds for individual topics (like https://www.cake.co/topics/bXGMkd1/) are somewhat hidden and seem to be sorted by their creation timestamp, not by the timestamp of last post(?). Maybe offering the user a better way to access individual topics they like, and giving them the option to sort existing conversations there in various ways, would be something that also solved this issue?