Steps Involved: 
- Open SharePoint 2010 Management Shell by going to Start | All Programs | SharePoint | Microsoft SharePoint 2010 Products | SharePoint 2010 Management Shell (Run as Administrator). 
- Run the following script.
Powershell Script:
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 ##Add new tag with the specified URL for the current user using SharePoint 2010 web service in powershell
 $uri="http://serverName:10736/sites/ECT/_vti_bin/SocialDataService.asmx?wsdl"
 ## $url is a string that contains the URL for which the tag has to be added by the specified user
 $url="http://serverName:10736/sites/ECT/"
 ## $termId is of type System.Guid that contains the Guid of the social tag term
 [System.Guid]$termId=New-Object -TypeName System.Guid -ArgumentList "99f1203a-a456-4948-820d-559d8fdc6702"
 ## $isPrivate -true to indicate the social tag is private; otherwise, false. If this value is null, this method assumes false for this parameter.
 $isPrivate=$false
 ## $title is a string that contains the title of the social tag
 $title="Tag Added"
 
 
 ## Web Service Reference - http://Site/_vti_bin/SocialDataService.asmx
 $socialDataServiceWebServiceReference = New-WebServiceProxy -Uri $uri -UseDefaultCredential
 $tag=$socialDataServiceWebServiceReference.AddTag($url,$termId,$title,$isPrivate)
 
 
 ## $tag is of Type: [SocialDataService Web service].SocialTagDetail()
 ##[SocialDataService Web service].SocialTagDetail() - Refer http://msdn.microsoft.com/en-us/library/websvcsocialdataservice.socialtagdetail_members.aspx
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