<
> I'm just a little curious about this issue, what if these people know each other, they often give each other merit? How to prevent abuse. It is like asking for merit

The conceptual principal is to be as objective as possible when awarding merits. That is, if you have real friends here, but their posts are not really any good, then they should not be merited. Of course everyones objectivity has at least a spice of subjectivity, meaning that one may be more prone to some groups of people than others, or at least to some types of posts than others. Its kind of human nature, but the objectivity principal should try to prevail.
Merit abuse is really more down the line of meriting each other for the sake of giving each others accounts merits. Now the forum has seen plenty of cases where one (allegedly) merits his alt accounts (which is considered unethical). That sets a precedent, and there are people that look for connections between accounts, being merits a possible strain of feasible proof. Of course, other may get caught in the crossfire due to these precedents, so the more prudent scenario is to always try to be as objective as possible.
One thing to consider is that merits are not moderated, but they are traced. Historically, some cases of very blatant abuse have been undone by @theymos, but the more usual derivative is that of getting sender and receiver potentially tagged (Trust tag, with negative review and rating). This is an on and off thing, and not everyone agrees to this use of the Trust System , but it does happen with more recurrence than one may think.