And We have enjoined on man (to be dutiful and good) to his parents. His mother bore him in weakness and hardship upon weakness and hardship, and his weaning is in two years give thanks to Me and to your parents, unto Me is the final destination. (Qur'an 31:14)
The literal word used in the Qur'an for "Thank" is "Ashkur" nothing but "Shukran" or the like.
people who have followed them that saying "Jazak Allah khair" is superior to simply thanking?
Your question is vague. I'll explain you how in contrast with today's world of thanking vs the "thanking" used in Qur'an 31:14 and also probably used in the article pointed out by Dinar in the comments. (https://plus.google.com/109702928551400026668/posts/VcqzMhdLe7e)
I think it is pretty clear in the article pointed by Mr. Dinar the actual meaning of the word "Shukr" or the actual "thank".
Below are the definitions
Modern (and rather stupid) thanks: to express to someone that you are pleased about or appreciate something that they have done (Cambridge Dictionary)
Actual (and rather useful) thanks: do something useful to the one who helped you.
The above actual "thanks" is what that can be inferred from Qur'an 31:14 and also from the Hadith in the article i.e. thanking, can be either done by doing something useful to the one who helped you, or simply making Du'a (may Allah grant you good things) for the person who helped you. (Still even in this way you're doing something useful to the one who helped you).
I don't see how today's definition of simply writing the word or saying "Thanks"
would even come closer to the actual "Thanks".
Hence, as a sense of awareness and general perception, put the above information in light, it can be inferred that if you're helped say, here in this site then you either make efforts to know the address of the one who helped by teaching you something and then send him a gift or do the like which is useful for him .........or......... simply make Du'a for the person if he deserves it as simply asking Allah "O Allah! Grant him something good." or in other words tell him "Jazakallah Khair" meaning/specifying, you already wished/prayed in your heart for his welfare.
Should we use “Jazak Allah khair” instead of “Shookran” or “Thanks”?
It doesn't matter whether you use the word "Jazakallah Khair" or "Shukran" or "thanks", unless you do something useful to the person who helped you. So, instead the one who helped you prefers something useful in return from you. So, simply using the modern "thanks" to please him would do him no good to please him in reality unless you specify, that you did something useful for him, i.e. by specifying that you prayed for him, i.e. by saying "Jazakallah Khair" (May Allah reward you with good things). After listening, to this, a person / believer / who is aware of what "Jazakallah Khair" means and it's importance would be glad to hear this than a modern "thanks".
May the creator guide us all.