- In Ukraine (similar to other ex-USSR countries) govt laws require from phone companies, to provide service "at least 8 hours after power off". To be exact, this is requirement of all phone licenses, land and cell.
So in real life, on most cell towers accumulators withstand much more - I have worked in telecoms, 24h of work without power is normal for them.
For smartphone power, in Ukraine most people have powerbanks, so also could tweet for 24h and more.
But when without power for few days or more, sure business activity fall and less people will be active on internet.
Also huge factor in this war, huge number of refugees. Even considering not all people flee abroad, but about 12 millions (from approx 40 millions) moved from east/north/south regions to west regions.
Look at a live twitter geolocation heatmap and it becomes pretty clear which areas are 'offline'.