Probably most of you know what it is, right?
It's the most popular SNS in Japan, I suppose.
You need an invitation from your friends to join the community.
I got an invitation looooooooooooooooooooooooooong time ago, but I didnt register because I didnt know what it was, then.
But when I met my friends from uni last Saturday, one of them told me that my friend who I had been looking for was in that SNS.
So another friend gave me his invitation and I finally found her!
She is the one who threw the birthday party where I met my honey! 
This SNS is very convenient to find someone but also very scary because someone I dont wanna contact to can also find me.
This has a function of traceability, so if you visit someone's profile page, this person will know who you are and vice versa, so you can trace those who have visited your page, too.
This is still useful to keep in touch with your friends, though.