Social plugin Facebook is not posting on facebook

There's a few possible reasons. First, when I try to run your page through the Facebook URL Debug tool, it comes back with multiple errors about scanning your page:

Object at URL 'www.glansbeton.be/nl/woonbeton/glansbeto...g/ecologische-bouwen' of type 'website' is invalid because a required property 'og:title' of type 'string' was not provided.
Warning
Curl Error : WRITE_ERROR Error while processing content unencoding: invalid block type
Warning
Errors That Must Be Fixed
Object Missing a Required Value Object at URL 'www.glansbeton.be/nl/woonbeton/glansbeto...g/ecologische-bouwen' of type 'website' is invalid because a required property 'og:title' of type 'string' was not provided.

Then, on your actual page, if I look at the HTML on your page, there are 2 sets of Open Graph tags. One set is from JFBConnect and the other looks to be hard-coded into the top of your template file. You should remove the extra tags.

I hope that helps get you going, but after you fix those issues, if you still have problems, just let us know.

Thanks,
Alex

Related to Alex's note about the problems with Facebook scanning your pages, please see our common support questions at www.sourcecoast.com/jfbconnect/docs/common-support-questions/ for "My Share preview is incorrect" for the most common solution to solving the error Alex was seeing.

Is it possible that K2 makes directly tags from the description, or how can I make tags in or from a article ?

JFBConnect provides a few different ways you can set the og:description tag.
1. For K2 items, you can enable the Open Graph - K2 plugin. By default, JFBConnect will then try to grab the description from the first words of the article text.
2. You can set up an Object in JFBConnect > Open Graph > Objects for a specific category if you'd like to specify that the description is generated differently than in #1.
3. You can add an easy-tag to the K2 item, like {SCOpenGraph description=This is a great article!}
4. If you don't do #1-3, JFBConnect will attempt to generate the description from the metadata description on the page, which K2 might set.
5. There is also a default description that you can set which is used if #1-3 aren't done. This can be set in JFBConnect > Open Graph > Configuration.
For more info, see www.sourcecoast.com/jfbconnect/docs/open-graph/tags-for-joomla

Natuurlijke materialen zijn weer helemaal trending

Can you provide a translation to English for this?

But still no posting on my own facebook timeline

You have another thread going about an error with your Facebook application settings. Until that is resolved, we should not address this, as it may get cleared up when we fixed the other issue.