It's a dilution of the *ownership*; the public used to own x% of Facebook and now they own less than x% of the bigger Facebook that incorporates Whatsapp (assuming that Whatsapp was completely private before).

In principle the $15 billion should be allocated proportionately between the existing stockholders (x% of it for the general public, y% for Mark Zuckerberg, etc), but it doesn't really make to think of it that way unless Whatsapp is actually worthless. What's important are the proportions. Suppose that the new issued shares correspond to 25% of the previous share capital. Then previously the general public owned x% out of 100%, and now they own x% out of 125%, i.e. (0.8x)% of the new share capital.

Whether the actual *value* of those stocks has been changed depends entirely on the actual value that Whatsapp adds to the old Facebook. As Dheer says, only time will tell on that one.