If you really want to be technical
OK, let's be technical. Let:
D be the number of depressed people; A be the number of internet addicted people; DA be the number of depressed and internet addicted people. T be the total number of people.
Then: "internet addicts are more likely than the general population to be depressed" means "the proportion of DA among A is greater than the proportion of D among T", or "DA/A > D/T", which is mathematically equivalent (since all number are positive) to "DA*T > D*A".
"depressed people could be less likely than the general population to be internet addicts" means "the proportion of DA among D is greater than the proportion of A among T", or "DA/D < A/T", which is equivalent "DA*T < D*A".
it is feasible that, although internet addicts are more likely than the general population to be depressed, depressed people could be less likely than the general population to be internet addicts.
No.
A general dissatisfaction with life seems to be one of the hallmarks of humanity and that is a good thing. We wouldn't be where we are if we were all content just living off the land like the other animals.
True, we would be
No, it doesn't. The summary says "more likely"; that is, as internet use increases, the probability of depression increases. That is the definition of correlation. Implying causation would be using a word like "cause". (I know, tricky concept) Which the summary doesn't.
The word "cause" would assert causation. When the summary says:
People who spend a lot of time surfing the internet are more likely to show signs of depression
it suggests causation, because that does not sound the same as:
People showing signs of depression are more likely to spend a lot of time surfing the internet
One might agree that those sentences are formally equivalent (in an idealized version of english), but the way most people speak, those sentences suggest different causations.
Even in a distributed system there is somebody at the top. There has to be, otherwise where do you start from a blank slate?
It is easier to write an incorrect program than understand a correct one.