18:00 UVT/GMT is probably bc Europeans are still awake then, and America is just beginning to get out of work but even at work they have access to phones/internet.
Monday might also correlate with people quickly tiring of work on Monday after a weekend, easing into the workweek. Dip on Wednesday would be due to peak-work-performance, before Thursday begins people taking longer weekends.
The "repeak" at 20:00 would be due to Americans REALLY being done with work about then.
Would be interesting to see the graphs parsed even broader down to the minute or 15 minute intervals.