I think that depends on the individual socialist. Truly selfish, one should know that one’s own well-being is intertwined with and best served by the well-being of others.
Then, people also differ in their altruism. As for me, I feel more for other animals than for other humans. WHAT??? I hear most here exclaim. But it is true that there are many, many people who are like me in this, if not as many as are not. Which is just to say we are all different in our altruisms – but agree about the need for socialism.