This is what I did before I wrote my last post.
Although the metatag admin panel (tiki-admin.php?page=metatags) has a switch to turn on/off »Freetags einbinden« (german i18n; I assume english i18n is »Include Freetags«), on a Tiki 7.0 installation I didn't manage to have the Freetags included as a page description in the META tags, no matter what.
I also noted during this testing, that it was not enough to stay within the usual edit-read-cycle of a wiki page to make the Freetags visible at all — rather I had to leave the page for another wiki page and come back to the original page to make the Freetags visible even in the document body.
It's also irritating that the metatag admin panel has another switch named »Benutze individuelle Seitenbeschreibung anstelle« (I assume it's »Use individual page description instead« in english i18n). The naming of the switch suggests that if this individual description is given, than it will shadow other descriptions — maybe also a page description resulting from Freetags. (I couldn't check this, because, as I wrote above, »Include Freetags« in the META data is entirely not working for the Tiki 7.0 installation I tested.)
»Use individual page description instead« is almost what I'm looking for; unfortunately it just puts the given string into a META tag of the name »description«.
Actually, the Open Graph protocol is not needed to make use of Facebook's »social plugins« like the Like button — Most of the social plugins will work without Open Graph data. (I already have lots of fun with the like button on a certain Tiki installation. )
The Open Graph data is rather used to attach an entity that is external to Facebook to Facebook's »social graph«, the giant datastructure behind Facebook's surface. External HTML pages which are tagged with Open Graph data can behave like original company or project pages inside of Facebook.
OK, I'll check now if I can include individual META tags using the field »User defined HTML content« on the Look&Feel admin panel (tiki-admin.php?page=look) by some include or link mechanism.