The fiscal sponsor must be a 501(c)(3) organization as recognized by the IRS. The fiscal sponsor cannot be an organization whose tax-exempt status is pending. The sponsored project must be able to qualify for exemption under section 501(c)(3), even if the project has no intention of ever applying for it.
Pressing the Ignore button on the listed invitation on the LinkedIn site is effectively a soft rejection. The message then moves to your Archived folder. You can later choose to open the message again and accept the request.
Facebook doesn't notify people when you unfriend them. However, the person might notice that his or her friends have decreased by one or that you're missing from the list. They could also be tipped off by visiting your timeline only to pull up a blank screen.