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See how it spikes up on the right hand side of the chart. This is probably the reason the mempool is almost cleared. Not sure who it is who has adopted segwit (one of the exchanges perhaps?), but it is a welcome development.
Great news. Mempool has declined for about a week now, even before the current spike of SegWit adoption, so I think it's the other way round. It seems more like people have taken advantage of the last few days of lower fees to move their coins to new SegWit addresses, so now a larger amount of people are able to send SegWit transactions. Keep in mind that for most of December Bitcoin was barely usable, so everyone who still had coins in legacy addresses was well advised to wait the congestion out before moving on to SegWit.