Has exercising with friends been shown to be healthier than exercising alone? e.g., is it better to run with friends or alone?

Yes: Studies have shown that having an exercise partner can lead to more consistent exercise as partners help to motivate and keep one another on track. In addition, it provides a social component to the physical ones gained in exercise. Choose a partner who is likely to keep going, and keep company you enjoy!

Depends: Depends on whether you're extroverted (get energized by other people) or introverted (other people tire you out). Exercise pals keep you accountable & you have a social reason not to skip your exercise. You gave your word! but if you find you like letting your mind go blank or use the time to think stuff through, a buddy would make exercise less enjoyable, unless they're introverts, too!