We've been asked before about this. Our less than earth-shattering advice has been to join a group of people with similar interests, eg a club or association, or an evening class.
Established clubs are a good place to meet existing residents of Hongkong. Signing up for beginners' classes in Chinese seems a good way to meet up with other new arrivals.
Last week's copy of 'The List' magazine was titled 'Get a new hobby', and had a good list of the different groups in Hong Kong. Their website doesn't have a copy of that issue, but they do have a directory of clubs and associations. Could be a good place to start if you're wondering what you could sign up for.
Here are some good network events
This is actually a very good question being asked over and over again. Here is a list of good event to go to this week:
http://www.localsolutions.com.hk/blog/2009/08/hong-kong-events-for-the-w...
keep checking this blog and they have some good consolidated events there