Facebook pays their interns over 5k/month. With how tight the market for CS graduates is, the idea of someone working for free for a company that makes a for-profit product is pretty crazy.

Unpaid (or, practically unpaid) internships are a sad reality of job markets where the supply far, far outpaces the demand - this is common in high prestige professions that do not require extensive qualifications - things like PR, fashion design, art galleries, magazine publishing, event organizing, etc.