This is a case where an anecdote is proof, at least a partial proof.
If the proposition is "Facebook brings at least some good to the world". A single confirmed anecdote of Facebook bringing a single good thing to the world is a proof. In the same way that all you need to prove that black sheep exist is to show a single black sheep.
The proposition is "Facebook brings a lot of good to the world" is a bit trickier, we need to define "a lot", but it can still be proven by "a lot" of anecdotes.
It is not a generality we are trying to prove, so we don't need to have data representative of the general case.
The part about Facebook being the best for dating is stated as a personal opinion.
If the proposition is "Facebook brings at least some good to the world". A single confirmed anecdote of Facebook bringing a single good thing to the world is a proof. In the same way that all you need to prove that black sheep exist is to show a single black sheep.
The proposition is "Facebook brings a lot of good to the world" is a bit trickier, we need to define "a lot", but it can still be proven by "a lot" of anecdotes.
It is not a generality we are trying to prove, so we don't need to have data representative of the general case.
The part about Facebook being the best for dating is stated as a personal opinion.