So, ELI5 level.
People who use Facebook also may feel depression, from very strong to none at all. In the middle of this interval there's the "expected value" point, sort of an average level of feeling depressed. This point is at an equal distance from the "most depressed users" group, and from the "not depressed at all" group. Let's call this distance of depression strength a "standard deviation".
Now, the users who stopped using Facebook became slightly less depressed, by 6% of that "standard deviation" range. If you buy a small coke at a McDonald's, then take one sip, you make it about 6% smaller. It's not unnoticeable (you've made that refreshing sip), but about 15 more such sips still remain!
In other words, there is an effect which can definitely be noticed ("statistically significant"), but it's not a big deal either.