Hi

Site Feed web part is not working in SharePoint 2013 online (Office 365 E3 version), when I add this in any page.

Any approach, information, code about this would be greatly appreciated. Thanks!

Hi,

Please make sure you have activated both the Following Content and Site Feed features on your site using Gear > Site Settings > Manage Site Features.

http://www.sharepointmaestra.com/Blog/Post/41/Where's-the-Site-Feed-Newsfeed-MicroFeed-Web-Part-

As this question is more relate to Office 365, I suggest you post it to Office 365 Forum, you will get more help and confirmed answers from there.
http://community.office365.com/en-us/forums/default.aspx

Best Regards,

Dennis Guo