That’s partially a case. When you search for a service, you can filter the amount by services by selecting a ‘Seller online’ tab. That’s probably one of the main reasons why you are being contacted more often when you are being active on a website.
However, staying all the time online in order to get a 100% engagement with a potential customer isn’t really a case. Your gig(s) are your main decisive point for a customer to decide if they want to engage you or not.
For example, I was offline for two weeks and then one day, people started to message me out of a blue. (Probably a rotation mechanism in play?) and people continue to PM me even when I am sleeping