First, lets quantify social mobility, so the entire discussion isn't about anecdotes:
http://www.epi.org/publication/usa-lags-peer-countries-mobility/
Second, lets remind ourselves of why the program exists: to create incentives for highly educated but high-debt professionals to take low-paying public service jobs (i.e. non-profit or state jobs). An example might be a lawyer from a top 10 law school, with $200,000 of debt, taking a public defender/ prosecutor job (salary $35,000) instead of the corporate law job (salary $150,000). This program isn't about social mobility, its about allowing the poor to access high-quality professionals.
Third, there are serious incentive problems with loan forgiveness.
Federal exposure to student loan debt now tops a trillion dollars, which is more than the entire country's credit card debt. Schools have little incentive to control price, especially if most loans are federal, and worst-case scenario could be forgiven by working at a non-profit. To see how one law school took glorious advantage of this, see here:
http://www.washingtonpost.com/blogs...law-gets-uncle-sam-to-pay-its-students-bills/
Finally, public service loan forgiveness is arguably being taken advantage of by doctors. Because we can claim residency as part of our 10 year repayment period (most hospitals are non-profits, and most for-profit hospitals create a separate non-profit entity to pay residents), those with longer residencies could get their loans forgiven merely while investing in their education to improve later profitability (ex. "I'll do a 7 year residency + fellowship, then only have 3 years of real service before getting 200k + forgiven).