Read the terms and conditions closely to be sure you don’t give up rights you can’t give. For example, Google Gmail’s Terms & Conditions includes this statement in section 11.1: “By submitting, posting or displaying the content you give Google a perpetual, irrevocable, worldwide, royalty-free, and non-exclusive license to reproduce, adapt, modify, translate, publish, publicly perform, publicly display and distribute any Content which you submit, post or display on or through, the Services.