I'm sorry to say I do not know a whole lot about this topic, but there's a few things I know boosted Valentine's day in the Netherlands.

I'm sure you're familiar with its general history in America and the UK. I'm not very sure about it's history in the Netherlands before the 1950s.

By the end of the 40s, flower export from the Netherlands was a booming business, it went really well, but the internal demand for flowers had almost come to a full halt. To do something about this, one of the American members of the board of directors of an overarching florist organization (Vereniging De Nederlandse Bloemisterij) returned from America and suggested making Valentine's day or "flower day" a tradition in the Netherlands as well. (a large portion of the flowers were being exported to America)
To do this, they gave flowers or a cake to people who had made themselves useful for the community and called this gift a Valentine's gift, sometimes handed to them by a person referred to as Brother Valentine.

Sending cards (anonymously or not) did not become customary until the 90s or so, when Valentine's day was given a huge boost, most likely by commercialization and globalization, it just blew over. We were a bit late adopting the tradition compared to the surrounding countries.

While originally, Valentine's day was more a friendship day than a love day in the Netherlands, it has been focused on love more and more, especially since the 90s. The friendship aspect is still very much present, though.

That's about all I can offer at the moment.

I'm sorry I can't provide you with links, this is what I remember from my Symbols and Rituals course I did find a something that explains what I just said, but it's in Dutch... I'm not sure if that'll be useful for you.

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