There's no real safety issues when it comes to HK. I would just take the usual precautions though (no different to spending time anywhere else in the world) - Watch your surroundings and belongings (and drinks if you're heading out) at all times. When you're out at night, I would catch a taxi after a long night out, even though buses do operate overnight, especially if you're not familiar with the area. Once you get the hang of catching the buses, then, I can't imagine you'd have any problems either though.
Many hostels and guesthouses in HK are NOT like the ones you would find back at home. So, not many have large communal spaces where you can meet up with travellers. You will really need to select wisely (in terms of actually choosing a place with the 'social atmosphere' you're after), so, that you don't get disappointed when you arrive. I've read reviews on Hostel World where people turn up surprised to find they've just been allocated a room and there's not much in way of organised social activities, etc, which really isn't that abnormal at all. Also, go in with realistic expectations in terms of hostel space - HK is not the place to go if you are looking for a spacious place to relax! Hostel rooms can be and generally are tiny.
I would suggest checking out Yesinn@Causeway Bay on HK Island. I know they have a garden terrace where you can meet up with other travellers. Location is close to the MTR station (makes your life easier!), and Causeway Bay is the perfect location for shopping. Possible downside - The dorm rooms cater for 9 people in each room. You are best reading reviews of potential hostels to look at on hostel booking websites, so you know the good, bad and ugly before making your reservations.
Avoid Chung King Mansions. The hostels in there may be cheap, but, that should not be the reason why you choose to stay there (many here will advise of the fire safety concerns).
I think you've chosen a great destination! HK is a great and safe city!
Edited: 2:38 am, October 05, 2012