Is it true that half the people who have ever been alive are still alive now? I find that impossible to believe.

No this is not true. It is estimated that 106.4 billion people have ever lived. 2004 global population was 6.4 billion, which is only about 6% of the total number of people who have ever lived.

Here's an excerpt from an interesting web site (link below):

"World Population: How Many Have Ever Lived.

An unknown writer claimed that "three-quarters of all the people who have ever been born are alive today". That erroneous statistic became accepted as fact. However, there is enough information to make a good guess as to how many people have ever lived on Earth. According to calculations a total of 106.4 billion people since man appeared about 50,000 B.C. That means that 5.8% of all the people who have ever been born are alive today. Every year, global population increases by about 78 million people. It is estimated that humanity is consuming the earth's resources 20% faster than they can be sustained. Until the modern era, world population grew slowly. During the next eight milleniums, population grew at .05% per year, reaching 300 million in 1 A.D. During the following 16 centuries, the annual growth rate fluctuated, partly because of the Black Death, which ravaged 14th century Europe. Today, there are six times as many people alive as at the start of the industrial revolution, 13 times more than when Columbus set sail and 20 times more than during the Roman Empire."

This is an excerpt from:
http://www.overpopulation.org/faq.html

They're still alive but on different life cycle planets, that's why there are so many stars!

Doubtful. Even with the old timey mortality rate, there were still alot of infant deaths which didn't really count for population statistically.

It used to be ten percent. Now it's apparently closer to six percent.