How to save? I haven't tried that and no idea so far
You have to send a data with AJAX (like JSON string...variables...etc) and use php to save the file or send variable to your database...
In my game, I save all of the data on my server, every level has the json file in the folder of the user, to save the progress of every level, and I store the coin, life, update of the user....etc in a database
1° make a registration page (very easy, just let's make an user, pass and choose a random ID), if it's for facebook, you make a registration at the start with his UID, username, email...etc
2° Contruct2, will send a data (user,ID,password,email...etc) via AJAX to a php file, and the php file will insert the data on your database, and also, you can make a folder from the ID of the user in your server.
3°now, the problem it's when save the data... for the most of the game you don't have to think so much about this, for example, for the most game, you can use a checkpoint, a savegame button...etc... so, very easy, and you can store the savegame of the level in your server (think about a tilemap, if you destroy a terrain, collect some special item...)
4° now, in my game, I send a JSON string from the Array, and my php file will make a file in .JSON in a folder of the user with the name of the level.
5° at the start of the game, construct2 will load the variable of the database, and check if exsist any savegame for the level, if there is a savegame, just load the JSON file.
just simple script of php, you can find everything on the web and in a official manual of php