Sharepoint Online – Unable to send new comment for Noteboard by programmatically.

You can get more information about handling comments for good old NoteBoard webpart as below
http://social.technet.microsoft.com/Forums/sharepoint/en-US/8023e4e5-ef21-40f4-b1ad-4c7b659598c6/retrieve-and-add-comments-to-a-noteboard-webpart?forum=sharepointdevelopmentprevious

http://msdn.microsoft.com/en-us/library/ff407953.aspx

Or you can use directly conntecting SocialComment web services by consuming “SocialDataService.asmx”.

But if you are working with SharePoint Online (SPO) , there is no more Server Object Model .And if you try to reach “SocialDataService.asmx” you will face
“Admin has disabled some SocialComment web services” error in your SOAP response message.

Noteboard web part is one of compatible feature from SharePoint 2010 can be used in SharePoint 2013 via UI but it is not supports newly added SharePoint 2013 programming models and APIs .You can get more information about new API model below.
http://msdn.microsoft.com/en-us/library/office/jj164060(v=office.15).aspx

+Server Object Model and exposed web service implementation (asmx) has been disabled as by design for SharePoint Online.

+NewsFeed feature and webparts can be suggested for replacement which is fully supported for new SharePoint 2013 programming model and APIs

“SharePoint 2013 includes new client APIs that you can use to work with social feeds, follow people and content, and retrieve user properties in online, on-premises, and mobile development. When possible, you should use client APIs for SharePoint 2013 development instead of using the server object model or web services. Client APIs include managed client object models, a JavaScript object model, and a Representational State Transfer (REST) service. If you are developing an app for SharePoint, you must use a client API.”
http://msdn.microsoft.com/en-us/library/office/jj163783(v=office.15).aspx