Question: I bought a tablet Samsung Nexus 10, it came with latest Android 4.3 installed. Android 4.2 and higher supports multiple user accounts which is awesome.

My question is, if let's say I have Facebook application installed on two accounts, does it install it two times (taking 2x place necessary) or is it efficient and it installs it once and changes only login settings for separate users?

Answer: 
The second user will have to explicitly install the wanted app (in your example: Facebook) – as otherwise all accounts' app drawers would be cluttered with a bunch of apps not wanted by everybody, except for those who installed them. But if the app already had been installed before by another account, it will rather be a "fake install" (i.e. it appears as if the app would be installed anew, but it's not downloaded again from Playstore, and thus doesn't consume "double space").

One side-effect to that is: whenever a user decides to *update* the app, that update applies to all other accounts of the device as well.
A couple of points for those swinging in from Google..

Uninstalling an app in one user account doesn't remove it from the other, unless you use Titanium or similar, in which case it is GONE.

If you install an app at the Play Store web site, it installs into the *primary* user "desktop", even if you are logged into another desktop using the exact same account on your Android device. Theoretically, it should quickly appear in your "My Apps" list, so it's easy enough to install into the second user desktop.

So if you installed an app and see no notification or icon, this is why. Switch to the primary desktop and there it is!