Hi Martin
Blogs can be effective when used correctly and within context, however a blog is just one of many social media tools.
Social media is a lot more than blogs. At its most basic t is about connecting people through user profiles, interest and user generated content. There are many other tools, applications and widgets available to allow the sharing of music, video, links etc.
There are a host of white label Social networking platforms available ranging from solutions for small-scale communities such as the one your Polish friend is looking for, right through to larger solutions designed for large institutions.
All have different approaches, rules and revenue models. There is bound to be one that suites your needs. The downside to using one of these solutions is the lack of flexibility in creating your distinctive look and feel. You are also tied to their set of tools and how they are laid out/function.
Here are a few to get you going. If none of these work for you search for white label social networking platforms. I have just selected a random sample of ones that I know provide the scale of services your friend is looking for. To determine the best solution would require more information from your friend as to the goals and business plans he has for this. The best solution for him may not be within this list.
http://drupal.org/
http://www.ning.com/
http://www.phpfox.com/
http://www.phpizabi.net/
http://www.boonex.com/
The key to this will be the preparation you friend does. It is not a case of If I build it, they will come. He really has to get thinking about the audience, their expectations, why they would use this, how to get them to join, what will keep them coming back once they have joined, how is he going to market it blah blah blah. Is there a revenue model or is this a community service?
Get him to read the small print of the white label solution he goes for. With some of them you have no control over the advertising being displayed on the site and you have no recourse to any revenue advertising on your site earns.
I hope this helps
Alasdair