When I used to work for companies here (I have been self-employed for over fifteen years now), it was normal that you didn't see your colleagues outside of the office. Unlike other countries, such as the UK where you may gather in the pub on Friday evening, such things are not done in the NL. Working in a company with many/majority expats could be an exception to this rule though.
When I came here back in 1991, I took on board my partner's friends. There were also a couple of work colleagues who we have stayed in contact with since moving up north. We stayed in contact with our neighbours from where we lived in the West but the contact grew less over the years.
We met most of our current friends through our children going to school here, when we all would drop off and collect the kids from each other's houses.
There are expat gatherings which still take place in the areas where the majority of expats congregate, but as Cynic wrote above, the Dutch form friendships at primary school onwards. Family comes mostly first and then the school-university friend 'kring' (don't get this word confused with 'kreng', which means something completely different). Many Dutch will also think, as you are an expat and here temporarily, that it would be a waste of effort to form a friendship with you.
Over the last thirty years, I have found it easier to form friendships with other expats, couples who are a mixture of Dutch and expat (or immigrant, including 2nd or more generation), immigrant families in general as well as Dutch singles/couples who are highly educated and/or who have traveled a lot outside Western Europe.聽
I met a lot of expats when I studied Dutch in Utrecht back in the 90's and I still keep in contact with a dear South African friend I met on a language course there, although she moved on to Switzerland many years ago now.聽 聽
Life can be lonely in the NL (through no fault of trying!) Contact with your neighbours can range from none existent through to meals together, but I suppose that applies to many countries.
But, if you have a Dutch partner here, there is always the mostly huge Dutch family that you will get to meet on every birthday occasion and celebration imaginable. Just remember to make sure the chairs do not get to form a circle and you get seated between people who don't (won't) engage you in conversation (I used to hide as many chairs as I could when I first came here in order to prevent that....!)