In the network tab in activity monitor there is a read out of data usage showing sent and received in GB. What this dosnt tell you is over what time period this is, is it from a day, a week and month ?
This is from OSX Yosemite.
For the top two figures, since last boot; for the bottom two, current.
After comments: It may be since launch of Activity Monitor - I have mine launch at boot & it stays open all the time, so I probably didn't notice the potential discrepancy.
The little graph is 'since switching to that tab', as it doesn't log more than about 5 minutes of data.
This machine's been up 9 days, this looks about right for my standard usage -
If I initiate a large download, the bottom figures become this [on a 150Mb/s line] -
The graph scale auto-zooms, so the earlier few KB/s vanishes into a flat line.
Once the file has finished downloading, the top figures show the increase of about 2GB & the bottom pair goes back to being relatively idle
The Data received and Data sent includes all network traffic. So if your machine is connected to a wi-fi network, but not the internet, it will show data sent between machines/devices. Also, if your machine is connected by ethernet to another machine, that's also counted. I figured this cos my machine (not connected to the internet) showed traffic of over 20GB, which tallies with the amount of data that I copied between this and another machine. So, this figure is not a count of internet usage data. As someone who occasionally has to use the iOS personal hotspot function, it's reassuring to know that this figure does not represent cellular data usage. For that, reset your iOS device's data counter.
The data received count is reset when you restart your computer