I would like a general "tell a friend function" where the visitors can send a message to a friend with a link to the startpage of the website. Not only to specific products or articles.

There are many third party programs that can be easily implemented.
 
I use www.addthis.com
 
It basically adds a set of bookmarking and sharing icons e.g facebook, twitter, stumble upon, email a friend etc.
 
I added the code provided to the header.html file (in templates) at the end of the code.
 
It offer tasking also so you can see which pages have been "shared" when and how many times etc.
 
Requires a quick registration for tracking.
 
Have a try

Sorry that should read "it offers tracking"

sorry forgot to mention....be sure to check the help pages re. adding the code to ssl pages as otherwise if placed in the header when going to checkout if you use ssl a warning message will apear in the browser.
 
You only need to change the code very slight for it to pull images etc required from a secure zone.