Is Git 100% social/public? Can code be private? And last question: is it secure enough to where enterprise companies (not just tech hipster startups) use it?
Git is also arguably more secure than many other VCSs since it (in a very very small and generalizing nutshell) works by creating a directed graph of SHA sums. You can't change a file and propagate those changes to other people without everyone being well aware of what has been done. Not unless you can compute useful code that collides with existing code... check out what linus has to say about that improbability some time^ ;).
No, security shouldn't be the issue for enterprise companies, though scalability is often cited as a potential issue. If you currently use perforce to a heavy degree, you might have some issues with going to git. This is not to say that git cannot work for very large companies, it probably just isn't going to work well if you put every project in your large company in a single one or two repos (as many companies do with perforce). It'll handle large projects fine however, it handles the linux kernel like a breeze.
^ http://osdir.com/ml/version-control.git/2005-06/msg00583.htm...
These days you can use git submodules which work quite nicely.