you can also contact friends of vietnam heritage to get on thier monthly e-mail list of activities - many expats and Vietnamese participate in a variety of cultural activities in and around Hanoi.
Check out the Hash House Harriers (I have seen notices posted). Keep your eyes open for special events notices on bulletin boards. Then if you are into jazz or other music check out those venues.
New Girl - as a newbie you can't go past a lazy Saturday/Sunday morning breakfast at Puku, the good bars are changing by the week so hard to say what is good. A lot of people going to Titanic and then there's always the second coming of Monaco that is meant to be happening this week - or has it already happened? The Hash is good, but often older and if truth be told they are a bit of a different crowd - although all great people. Anyway, good luck. How long have you been here for?
Go to bars like Funkey Monkey... or bars in hotels. There's also a salsa dancing every Saturday evening in the "Nutz" bar at Sheraton hotel if you're interestd in.
For bussiness purpose- you can go to events of some organization such as Amcham- American Chamber of Commerce in Hanoi; they have several events during a month and you can register to join.
There is a party hosted by Funky Monkey at Nutz Bar, Sheraton Westlake next Friday. Try Labrynth Bar on Ta Hien, Polite Pub, Funky Monkey, Phuc Tan Bar & Titanic on Red River.