你吃了吗？（ni chi le ma)

在吗？

在干嘛呢？

最近怎么样？

This is an excellent question - and my short answer is that, we don't. In English, it's perfectly fine to say "hi" and "hey" and similar phrases, but in Chinese, if you say "你好", unless you're in an extremely formal situation, it will elicit strange glances. 

Usually, you just start off with maybe a nod in the other person's direction, a wave, and dive straight into the conversation as if it had never broken off from last time. Of course, if that "last time" is like 10 years ago, then something like "好久不见了！" may be appropriate, other wise, the convo just flows on. 

Another common ice breaker or convo starter is “吃饭了吗？", I remember when I tried that in English, I got a strange stare in response. Nevertheless, it's very effective and the norm with Chinese people. 

"最近怎么了？" is another common one, if you haven't seen a friend in a while (but maybe not that long, like 10 years or something). 

Other than that, I can think of no commonly used greeting. 

你好 hello，hi

干嘛呢 what are you doing

-在？Are you on line?

-恩，什么事？Yes?

-有钱吗？借点我。lend me some money

-不在。I am not on line.

  I never say  "What's up" to other people.