Can i ask? If i give merit to others my merit will reduce? Example i have 100 merit and i give 1 merit to one people and my merit will become -1 so my merit will become 99 ?
No. Merit points just increase (what you need to rank).
sMerit will decrease if you use it and you will lose nothing in your Merit points shown under your name (points that you could send to others if you wish "for free")
If everyone decides to not use this merit system because he has no time or will to click the link, even if he likes so much one's post, everyone's rank will stop levelling?
Afaik yes (considering that your merit at that time won't allow you to get a new rank).
And then someone change one lovely topic to legendary +00 day!
Edit : LoL Lutpin (You are a merit source)
I suppose : theymos, Cyrus, hilarious, yahoo are merit sources. (For the others including lauda and some Legendary accounts Ecuamobi, ognasty...etc it is quite possible)