My opinion is any kind of act that adds value to the society is wonderful , every social leadership starts with a selfish motive which is the universal truth.......
when it comes to MBA, if you read the admission blog of the adcom director of ISB, india he regretfully mentioned that students are trying to fake or involving in social service or charity few months before the application just for the sake of the application which is disheartening.....it questions your integrity and shows up oppurtunistic attitude......a charity or social service add value to your apps but not a necessity in any mba applicant profile and moreover definetely not a ticket into the college......most of the admit profile doesnt have any such charity or social service but as part of EC may have a strong hobby with some achievement in that area etc....my advice is only do it if you have passion and bring that passion out in your application if you want to bank on.
To sort of pile on... one recurring topic between me and my mentor as I try to decide on next career move (if b-school is not an option) is "do something you are passionate about" ... and I have figured out that I tend to have exponentially higher output when I truly care about something, which translates into success and leadership.
As far as late pile-on's... I had a similar dilemma. On Labor Day weekend I joined a professional organization (which is very new, pretty small, and somewhat exclusive) and I've had an impact working in that organization on top of my regular work. I quite literally tripped over it while working on my linked-in profile and it's been fantastic. I have put it on my resume and mentioned it in two applications, but I didn't talk at length anywhere for fear it might look like I just "joined to have something extra". It's been a great source of potential career leads as well.
Anyhow, my recommendation: My mom has a great quote about success; "80% of success is endurance" ... it is *not* hard to come up with a good idea, but it takes a lot of long hours to make said idea a reality. Pick something you're really passionate about, and make it a reality - but do it because you care, not because it fills out your app. If you interview and get asked about it, I'd bet folks will be able to tell the difference!