Can giving back to my alma mater have a positive impact on application? I going to create a really simple social network for my alma mater so that it could help people network better. After getting some work experience I was thinking of pursuing a masters degree in Computer Sciences. The school I graduated from is not a tech-school but best for arts and commerce in my country. 

So I was hoping that it could become a stand-out factor in my application. I have an average percentage.

EDIT

The degree I graduated with was not a CS degree but a degree in physical sciences. Further, the undergraduate school as I mentioned did not have a "proper" CS department.
Graduate admissions is based mainly on the question: Does this candidate's preparation suggest that he/she will be able and likely to succeed in our program?  It is not awarded as a token of gratitude for service to the institution.

This project of yours could have a positive impact on a graduate application, to the extent the project itself is evidence of your readiness for graduate work (programming skills, initiative, etc).  But from this point of view, it's not really relevant that you created it for your alma mater, or that your alma mater uses it, just that you created it at all. 

(The fact that your alma mater actually uses it could have a small effect on your application at that same institution, if some of the people on the admissions committee are personally familiar with the product.  They would have a better understanding of what the project says about you as a potential student.  Of course this could cut both ways, depending on whether they think the product is good or bad.)

I would guess that this sort of thing would be a relatively minor factor in considering an application, and would not probably not save an application that was otherwise weak.  But of course it will depend on your particular institution, their criteria, and the overall strength of their applicant pool.  You could get a better sense by having a talk with one of the professors in your department who is involved in graduate admissions.