$siteUrl = 'some-site-url'
$site = Get-SPSite $siteUrl
$serviceContext = Get-SPServiceContext($site)
$msstm = New-Object Microsoft.Office.Server.SocialData.SocialTagManager($serviceContext)
New-Object : Exception calling ".ctor" with "1" argument(s): "UserProfileApplicationNotAvailableException_Logging :: SocialDataManager.SocialDataManager Proxy has no ServiceContext available."
At line:1 char:10
+ $msstm = New-Object Microsoft.Office.Server.SocialData.SocialTagManager($service ...
+ CategoryInfo : InvalidOperation: (:) [New-Object], MethodInvocationException
+ FullyQualifiedErrorId : ConstructorInvokedThrowException,Microsoft.PowerShell.Commands.NewObjectCommand
Not much out there, but the UserProfileApplicationNotAvailableException provided a clue. I did not track down the specific cause, but it looks like I was not running powershell with an account that admin permissions in the UPA or MMS. I did not have time to dig further, but instead simply opened up my powershell as the farm account and did not run into the same problem.