In fact, as previously mentioned (quite a lot), there is activity, but not visible one (or at least not visible in terms if you don't go around and visit random user profiles, which almost no one would do). Assuming from stats that there are quite a few users active over a month, most people just use HB as an anime tracker, and honestly, even if Hummingbird is a social network-like anime/manga tracker, no one is ever forced to interact with the Feed or Forums, following no one at all.
With a growing userbase, activity in the forums/feed/groups may rise, but no one can guarantee that, of course.
Generally, as also previously mentioned, the big problem is the Feed and the Forums, people might think they already are active posting stuff on their feeds with a few followers they gained from going around through the forums after joining the site. Forums are mostly a place which, if not directly incorporated with the site (or being main focus), end up with low activity, since...well, it's the age of Instant Messengers, all sorts of other things that don't require you to write more than you want to. People mostly feel (and that certainly isn't that wrong) that a forum post should at least have some content, while a chat message can be discussing in the probably weirdest ways.
With this I mean is, that a feed isn't made for 300-word like posts to interact with. You can discuss smaller things, tell the world what you just did, have group discussions (as they only work on the feed), but as most can see, the conversations about on-going/finished anime mostly takes place here on the forums, as it is much better suited for such.
The probably best solution would be trying to find a way to get the main site and the forums closer, like with unified profiles and notifications, as long as Discourse somehow allows access (well, it already does, example for this is Tenpenchii giving you forum notifications) to user notifications and other stuff over the site.