I think a key difference between Discourse and Twitter/Facebook/Snapchat etc besides operating at different scales, is that the “social” services require you are logged in to consume content.
Most Discourse sites on the other hand are perfectly usable as anonymous. This allows lots of people to continue using the site without tracking them as “active users”.
This leads to situations where some people are actually “active users” but are not being tracked. A classic example is mobile where often people just find it “too much of a pain in the behind to log in”. (also apps do not solve this)
There are things that we can do in software to push up some of these numbers, most of the non-evil things may give stuff a tiny bump.
(good) We can make it easier to login in mobile, in particular, “login via email” feature (you don’t require password, just send an email with a 1 time login activation)
(evil) We can take away the cake after 10 minutes… there would be enormous backlash in communities that do this.
(evil) You can hide large amounts of content from non-logged in users (and still make it Google visible) - also hated
(bad) You can encourage “general off topic” discussion and favor it over on-topic discussion. Turning site into a place anyone can visit anytime to kill time and talk about anything.
I did just run an interesting report on your site:
SELECT sum( case when time_read < 10*60 then 1 else 0 end ) less_than_10, sum( case when time_read > 10*60 and time_read < 60*60 then 1 else 0 end ) f_10_to_60, sum( case when time_read > 60*60 then 1 else 0 end ) more_than_60, count(*) from user_stats s join users u on u.id = s.user_id where u.created_at > now()::date - 365
So… 1992 people signed up.
659 stayed around for 10 minutes reading time.
684 stayed for 10 to 60 minutes
647 stayed for 60+ minutes
This ratio is actually quite spectacular compared to most of the sites we run. You are bringing on a regular amount of “fresh blood” into the forum. The reason you are only seeing 30% growth in DAU in the last year is simply because only a rare number of the 647 users become “daily” users of the site AND there is churn of old users that become less active and leave. (also … keep in mind the graphs you linked from facebook are not should much more than 30% growth in a year)
There are some things we can do in software to marginally improve things, but any radical change and growth really needs to be driven by the community and community promotion.