Is ethical approval needed for online studies?
The answer to the question "do we need ethical approval for X study" is ALWAYS yes.  The format is irrelevant.  Your institution's ethics review committee will have a category for "minimal or no participant risk" which will often have fewer detailed requirements for submission in instances such as the one you describe.  

Nevertheless, the issue is about /independent/ assessment of ethical research.  Even when you believe your study guarantees /zero/ risks, you must have an independent panel validate your method.