Workplace relationships...avoiding fraternization?

I have a hard time defining a line between what is professionally friendship and fraternization, and I believe my effectiveness with my job is being affected. My job is white collar but at the junior end of the hierarchy. I do not think it is professional to be solely going out with my fellow junior white collar co-workers on off times. But am I wrong in thinking that? I mean, there is a line between work and play, and I should not be putting them both together, right? The others do it all the time, and I am seemingly left out at work. Insight? I think it is necessary to have and be involved in activities/friends totally separate from work.