Nice topic. Just talked to my co-founder about that. We are developing both web service and mobile app, so there are to different views on the problem.
We didn't really thought about the whole problem of calling the user "you" or "me" yet, so here is where we are today:
In the web service we use "you" everywhere. So far so good.
But for the mobile app (we just finished the first prototype - without beutification and such) we used "my" in almost every case... and we investigated why: For us it feels convenient that if I hold my cell in my hand, it is MY phone.. and such is the mobile apps that I use. They are MY apps, and as MY apps, they are a part of me (I take them with me everywhere I go - just like my keys). In the keys-example you would also say "my keys" and not just "the keys".. and you say "my phone" and not "the phone". I think it is just intuitive that it is "my app" and (inside of the app) "my posts" and "my messages"...
In the web service we use "you" everywhere. So far so good. But for the mobile app (we just finished the first prototype - without beutification and such) we used "my" in almost every case... and we investigated why: For us it feels convenient that if I hold my cell in my hand, it is MY phone.. and such is the mobile apps that I use. They are MY apps, and as MY apps, they are a part of me (I take them with me everywhere I go - just like my keys). In the keys-example you would also say "my keys" and not just "the keys".. and you say "my phone" and not "the phone". I think it is just intuitive that it is "my app" and (inside of the app) "my posts" and "my messages"...