Always check the accompanying license before using a FontStruction in a project. The license determines how you may use the font. Many of the licenses used on FontStruct do permit commercial use. You can read a summary of the license on the FontStruction's homepage and it is included in the FontStruction download .zip file.
Remember that all of the licenses currently in use on FontStruct do require you to credit the original designer in your work. How you do this depends on the medium you are working in.
If in doubt, it is wise to ask the original designer directly by going to their homepage on FontStruct and clicking on the contact link next to their avatar.
Whether your project is commercial or not, a designer is also likely to be pleased to hear about when and where and how their work is being used.
Please contact us if you can’t find the information you’re looking for here.