If you 'like' only one post in a thread, yes, it follows that you don't 'like' the others. But you don't necessarily dislike them. You might even like them without 'like'ing them - which can also occur with your social example.
I tend to 'like':
Answers that I would have given if I had got there first - I am showing my agreement.
Answers that tell me something I did not know - I am saying 'thank you'.
Correct answers from non-teachers who have said that they are not absolutely sure - I am saying, 'that is correct'
Witty answers
Answers that correct a slip I have made, or which add something useful that I have overlooked
etc - that's a cop-out for the likes I give that don't fall into those categories.
That's all very well, but as nobody knows my reasons, or didn't until now, perhaps it's not too sensible.