But the trend in net migration (international+domestic) for the region works out to this:
The breakout for the +3,368 net migration for the MSA is +590 net domestic migration (movement within US) and +2,778 net international migration. So you can say in a sense that international immigration is responsible for most of our ability to bring in more people than are leaving.
But here is a factoid to ponder. Only 11 of Pennsylvania's 67 counties had positive net domestic migration estimated for the most recent year (2012-2013). Of those 11 counties, 4 were in the Pittsburgh MSA. Here are all the counties in PA with net positive domestic migration and the estimate. Beaver County doing almost as well as Butler, which is worth looking at some more. But at the end of the day, only Cumberland County in south central PA is really attracting a palpable flow of folks moving in from elsewhere in the US. Is this surprising?