I bet he's got no life and FB is his social outlet. Thing is, the busier and more popular a person is, the less time they spend online.
This guy does all these updates because he's got nothing better to do. And he probably 'likes' everyone else's photos as quickly as he does your girlfriend's photos because he wants to be popular and have friends. He's probably put her down as his 'sister' when he was bored.
She's saying "yes" to his request to catch up and then ignoring it because it's easier than telling him that she doesn't want to hang out with him. I once knew someone who I used to do that with....she'd say "Oh, we must catch up sometime" and I'd reply "Oh, yes that would be fabulous" and nothing more would ever happen because I wasn't interested in actually spending time with her.
Please think twice before you get all jealous over Mr No Friends who lives his life on FB.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.