If you want to figure out when your users last logged on, perhaps to clean up licenses in use by dormant accounts, the following Powershell code may help you.

########
#checkLastLogonTimes
#Copyright: Free to use, please leave this header intact
#Author: Jos Lieben (OGD)
#Company: OGD (http://www.ogd.nl)
#Purpose: Generate a CSV file with last logon times of all Office 365 users
########

$csv = "c:\temp\LastLogons_$(Get-Date -format dd_MM_yyyy).csv"
$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session
$users = get-mailbox -ResultSize Unlimited | select UserPrincipalName
Foreach ($user in $users){
$mbx = get-mailboxstatistics -Identity $($user.UserPrincipalName) | Select LastLogonTime
$upn = $user.UserPrincipalName
if ($mbx.LastLogonTime -eq $null){
$res = "Never"
}else{
$res = $mbx.LastLogonTime
}
$outStr = "$upn,$res"
Out-File -FilePath $csv -InputObject $outStr -Encoding UTF8 -append
}