How to store something like Facebook Groups in DB?

On Facebook, you can join/become a fan of groups. How do they keep track of things like that?
For my use, I'm looking at having users become a fan of different things.
Would you store it like this in the database:
Table: Fans
Columns: User_ID || Fan_Of (ID of group is stored here) || ID (Primary key? can't see ever using this field for any output...)
Row: 1 || 3 || 1
Row: 1 || 5 || 2
Row: 1 || 4 || 3
Row: 3 || 9 || 4

Where each time a user becomes a fan of something, they get a a new row in the table.

Is there a more efficient way of doing this? Thanks in advance!

P.S.
In the same topic (I think), would you store comments on a article the same way?
Table: Comments
Columns: User_ID || Article_ID || Comment || ID (Primary key? can't see ever using this field for any output...)
Row: 1 || 3 || "Great article!" || 1
Row: 1 || 5 || "Good job!" ||2
Row: 1 || 4 || "Impressive!" || 3
Row: 3 || 9 || "Good job!" || 4

 

you wouldnt need the last column if 'Fans', just make User_ID + Fan_Of the primary key. For Comments wouldnt Article_ID be the primary key?

 

Article_ID couldn't be the PK, no, because there would be more than 1 comment on an article!
I forgot about composite keys (Duh!), thank you! For the Comments table, adding a datetime column could make my PK User_ID+Article_ID+Datetime the PK.

Thanks!