[@Greenie]

In this case, you want to go for the guilt-trip without sounding passive aggressive, or at least that's what I'd do. Hit them with the: 

[i]'I really enjoy playing with you, but I worry that you're not as into it as you could be. Your posts seem to end abruptly, and I can't help but feel like you're rushing through them to get to something else. Is this a bad time for the RP? I understand if so, we can table it until you're free to give it more attention.'[/i] /sad violin music

And if that doesn't work, bail. Because you can get new friends, man. You can't get back wasted time.