Idealists have a lot of passion and can picture something bigger than themselves and the current marketplace or cultural reality they're living in. They dream big and have high level, sometimes lofty goals that they're always working toward. While a realist usually focuses on the more immediately achievable and has a bias for results, I don't believe the two have to be mutually exclusive as it comes to social entrepreneurship.
One of the most successful startups in my city is a Nashville style chicken/homestyle food restaurant that employees formerly homeless and incarcerated citizens and focuses on not only giving them a great place to work and learn tangible business and customer service skills, but also on teaching them life skills ranging from financial planning to health and wellness. The founder has been vocal about his big picture, idealistic vision of creating a much bigger more scaleable career services model for those who have struggled in the past after incarceration, financial troubles, health or family issues, etc. He realizes though that by proving himself with a successful business with a great product that customers want, he's able to foster that vision, prove the model is successful, and then continue growing it. He was realistic about his capital investment and didn't want to spread himself too thin at the beginning so he started with a food truck. He since has opened a very successful retail front, and is now expanding to another retail location.
Visionaries and idealists can also be realists, and are wise to test, improve, and gain other followers and evangelists versus trying to go after the whole sky without the right support network and loyal following. By remaining patient but fiercely passionate, and by continuing to always take forward steps (no matter how small) to achieving a greater vision and goal, you can continue to be an idealist without abandoning reality and ration. Having the courage to keep going after your greater vision day after day, even in the face of the often many challenges and roadblocks that get in the way (the reality) is a quality that sets the entrepreneurs that success apart from those who don't.