After growing its financial advisor network to 5,800 last year, MassMutual has continued expanding its workforce. In February, it bought rival MetLife’s network of 4,000 life insurance agents for $300 million. That expansion appears to be paying off. Sales jumped 14% last year, pushing MassMutual up 18 spots on the Fortune 500. Still, low interest rates and market volatility contributed to a slight decline in the insurance company’s assets under management and fee income from overseeing those funds. Overall, though, profits rose 7% for the Springfield, Mass.-based company, not a bad performance in a tough market for insurers.