The only way you can get all the information you want (in bulk form as all have stated) is login to your “Control Panel”, “Remote Desktop”, or whichever, and execute a backup of the database. Zip the file as it will be large (backup.sql) ftp to the site, and download both the zip file along with, the wp-contents/uploads, wp-content/plugins folders…
Once you obtain the base files, utilizing a tool such as “Filezilla” you may periodically backup the above folders while, “Filezilla” (for example) will save you time by not overwriting previously downloaded materials (photos, etc)…
Next, you will need to do a rather scheduled backup of the database albeit, via MyPhpAdmin, MySQL tools to maintain the “knowledge” by your site (lack of better words) as to which plugins are active as well as, which users are still active relative to their posts.
BEWARE… you will need to make adjustments to php.ini file to allow for MyPhpAdmin to “Restore” a large backup in case of disaster.
But to answer your question as Andrea has already, no there is not a plugin…
You may want to go out to the net and see if you could hire some php/mysql guru, that could create a script that would allow you to execute a database backup of each individual “blog” of your site via (wp_1, wp_2, wp_3, etc. tables) of course, the usermeta is crammed into one large file however….
If you are allowing each of your users to create use their own domains, I would suggest strongly simply creating individual installs of WP 3.13 for each user… this of course will create several different databases within your MySQL server, an equal amount of wp-content/upload and /plugin folders but, it is the only way at this time to achieve what you are looking for…
Good Luck!!!
P.S. You know on second thought, via MyPhpAdmin you could actually make one large back-up of the entire site, and then do individual table backups of the corresponding blogs… e.g. backup tables, wp_1 (all), wp_2 (All), etc.. separately and in need only have to restore a minimal number of files… see if there is a MyPhpAdmin guru out there that can give you a hand with a script…
