* timezones, most of the world is not on EST. If you make it per timezone, then not everyone will be on at the same time.
* 7.39 to 10.39 is not practical for everyone even on EST timezone. I go to sleep at 8pm, this means that I avoid screentime 7-8pm because of blue light. We all have different schedules, I tend to only use my computer during the business hours. In the evening I want to spend time with my kids, not be on social. Even if your social network would have a reward, for example 100$ a month to use it from 7.39 to 10.39, I would decline.
Some food for thought:
* Make the time open only based on a user selection. For example I would chose 9am to noon. Only allow switching time once a month. Clearly display a schedule of when most of your friends have selected a time, so that if I see that actually a lot of my friends have chosen 1 to 4pm instead, I can switch to that.
* Most likely after this comment, I will never see your site again. I would suggest adding a demo or a video, for people logging in outside of the hours, to entice them to come back when the site is open.