Quite simply a paid service stand out because of the remarkably unique features provided.
I'm reading your statement
as - As a owner how do I decide whether I should charge for the service or not?
How to decide the service is paid or free?
For that, your best bet is to compare your service with other similar services (and/or take other people's views for the same. What do others think about your service?). Like if we take example of Careers.SO and compare it with some service which is free (for recruiters also). So, while making the decision whether to charge or make it free, you'd consider that Is your service substantially better (in some or the other ways...preferably multiple ways) than the other free services? I'm assuming that you'd agree how Careers.So is better than other free services.
Ultimately it's comparison of features, and it's very much relative so even if today your service can attract lots of users who're ready to pay for, still if a competitor comes tomorrow and provides his equally excellent service (if not much better) for free, then it might be a good idea to reconsider your initial decision.
Hope that helped.
You have to devise a model for making revenue. For free model you could offer some paid options, services or system of ads, for paid service you could offer some added value and collect funds directly.
You may start a totaly free model, which should be viral in some way and then think over some paid models.