On websites like Internations and MeetUp, you can easily find other people that share your interests or background. Meetup also includes a page for the Expat Republic Rotterdam, one of the expat communities in Rotterdam. Furthermore, several organisations host events for expats on a regular basis. Venture Café for example, organises events every Thursday evening with the goal of connecting the local and global innovation community.
Find a list with more communities and activities here.
