On newer/latest versions:
Go to the address bar and click on the menu sign that is, the 3 dots and you will see share, then select one of the options you want to use.
On Opera mini 6.1 and 6.5:
Select Menu
Scroll down to Share
Click on the share and you will see, Facebook, Twitter, and My Opera
Then select one of the options.
On Opera mini 4.3 and 4.4:
Select Menu
Scroll to Tools and select it
Then select Share page
Click on the share page, you will see, Facebook, Twitter, and My Opera
Then select one of the options.
Note :
You can not Share the start page of Opera mini, you must first be on a website before you can share.