If everyone decides to not use this merit system because if 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?
Yes, however this is sort of like saying "If everyone stops using the trust system the nobody will gain trust ratings." While this is the case, it is a moot point, because it is not going to happen.
There are sources, and these people will continue to use the merit system even if they are the only ones doing so; this will of course monopolize merit in an unfortunate way, so it is in everyone's best interest to use the system as intended, rather than ignore it as if it were never mentioned.
This will makes ranks more valuable, disincentive spam and I highly doubt the 3-seconds it takes to merit someone will deter almost anyone. Are you that constrained for time?
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 does not decrease; the merit you send comes from your sMerit, which is a separate count; you have an initial amount determined by your rank and you acquire more by receiving .5 sMerit for every 1 Merit you receive.