How to share content and files with other users within the site (not in facebook or other social media)? Users would like to be able to share content with other users within the site (not on Facebook or any other social media).

For example, if I read a blog that I like and think my colleague would be interested I could click 'SHARE' and send it to another user's profile or work e-mail address. Or tag someone? This would require a notification to the user that they have shared or tagged in something.

It would be great to be able to do this with blog posts, files, photos, and videos. "
Besides the custom approach identified above, you may also want to look into using the Forward module.

https://www.drupal.org/project/forward

It includes some basic tracking for reports as well.