Get-MailboxStatistics will give you the last logon date, but not the user. For that you need auditing enabled on the mailbox. For last sent/received message, you can use the message trace, Alan has a nice script here: https://gallery.technet.microsoft.com/office/Office-365-Mail-Traffic-afa37da1
The new reports in the O365 portal dont include data from shared mailboxes, but you can still use the PowerShell cmdlets to get some additional info (for example the Get-MailboxUsageDetailReport).
And of course there are 3rd party tools that can give you more data (or historical overview of the usage)
