[ynniv]

Yes, OAuth tokens contain the Facebook id of the user. It is unlikely that this would matter since session tokens (required for most permissions) expire after an hour (no reason to store those) and sessionless tokens are per-application not per-user.

However, obtaining the application credentials for Instapaper would allow you to limited access to a user account on behalf of Instapaper. Since the credentials for doing this are per-application, Instapaper would likely store your Facebook id in their database. Given a Facebook id, you can easily see the account name by visiting https://graph.facebook.com/$id.