It can't look like obvious propaganda; people wouldn't use it. It doesn't have to convince people of any one thing to work anyway. Just shortening the average attention span would be enough to weaken a society a little. How about increasing polarization? People produce plenty of polarizing content on their own; just favor that a little more than an algorithm merely trying to be addictive does. Antisocial and self-destructive behaviors should also get a subtle algorithmic boost. Aggrandizing the platform itself is harmful as well - if someone chooses to be an influencer instead of a scientist, damage is done.

If those all sound like things most social media does, that's the point. It's like popularizing a junk food that's just a little higher in sugar, fat, and salt than the rest. Harmful elements of social media are harder to measure than that though.