From Inside Facebook as summarized from The Economist:

The average male Facebook user with 120 friends:

• Leaves comments on 7 friends' photos, status updates, or wall
• Messages or chats with 4 friends

The average female Facebook user with 120 friends:

• Leaves comments on 10 friends' photos, status updates, or wall
• Messages or chats with 6 friends

The average male Facebook user with 500 friends:

• Leaves comments on 17 friends' photos, status updates, or wall
• Messages or chats with 10 friends

The average female Facebook user with 500 friends:

• Leaves comments on 26 friends' photos, status updates, or wall
• Messages or chats with 16 friends

In other words, Facebook users comment on stuff from only about 5-10% of their Facebook friends. And as has been shown by many other studies, women communicate with more people in all cases than men.

This is the takeaway, and it’s very true:

His findings: while many people have hundreds friends on Facebook, they still only communicate with a small few. Or to quote the author of the article, "Humans may be advertising themselves more efficiently. But they still have the same small circles of intimacy as ever."