I use Kdenlive 0.9.2 under KDE 4.9.5 (Kubuntu 12.10).

Recently I published a video an Facebook and the results were disappointing specially the text. I tried different formats but even if the rendering-result looks good in VLC or Dragon Player, I couldn't read the text neither the video looks acceptable.

Which settings must I use for rendering?

What mistakes may I have done? (I'm not realy used to digital editing)

Does face book offer different resolutions for playback? Because I had a similar issue with youtube until I realized I could ask for a higher resolution video from the site

Have no experience with FB, but most sites that host video will convert uploaded video to their "standard" format (or a few of them, as ttguy says).
It is usually not possible to avoid that conversion and re-encoding, but by rendering to something as similar as possible to the site's "standard", you can at least minimize the amount of "surprise" after upload.

Try to download some video from the site, then use something like ffmpeg -i, or just play it from the command line with mplayer, and look at the console output, to see what is the resolution, codec, bitrate, interlacing, fps, etc...

Thanks for your tips! Unfortunately I got no eMail-notification though there is an subscription for this thread. :(

As far as I know, there is only one resolution on Facebook. Sad to say but I don't know how to download a video from Facebook so I can't check witch one it is. And until now I have now other account like Vimeo or youtube - because till now I host my videos on my little server, just for my friends.

I ain't hard to get a youtube account !!!. Then you can just post youtube videos to facebook.

You could try a site like http://www.keepvid.com/ to see if it downloads facebook videos

I sometimes use dwhelper
https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/
for Firefox, to download video, or else just look for the temporary file with the video. It used to be in /tmp, but now it is somewhere in /proc....
Just search for files bigger than a few MB, created less than 30 mins ago.