Technically, you can place the files wherever you want.
The Unix/Linux filesystem layout standard (FHS) includes specific uses for where things go, based on the idea that certain directories could be mounted read-only, some intermittently available, some shared between systems with different CPUs, etc..
Based on the standard, /home is for user directories and programs shouldn't rely on it. /usr is read-only, and /var is for data that might change.
My recommendation: Use the directory your distribution uses. In the case of virtualhosts, directories underneath it. That ensures the least likelihood of causing a bad interaction with any other software on the system.
Ubuntu uses /var/www, so use /var/www/example.com. Work with your Linux distribution, not against it.