> why does everybody say it's a "Facebook currency", a Facebook project, if there are +/- 30 corporations in the association?

Because FB launched it, and currently appears to have the most control over the project. At the same time, people don't actually take the time to understand the governance model.

> Does Facebook have more decision power than Visa, MasterCard or any other participating corporation?

No. They get a seat in the governance organization, which they hope will be about 100 organizations at launch (so they get ~1% of the vote.)

> Or is Facebook simply the main vector Libra will be promoted with?

For now, looks like it.

> Would Libra still be a project if Facebook was (hypothetically) shut down?

Yes, it would. Again FB will have just 1% of the control at launch.