NO. NEVER.
There are several privacy concerns associated with those, things I would like to avoid.

Like what? AFAIK, all it would do is post to your Facebook wall / Google+ page that you like HLP. Unless HLP was set to request permissions to post on their walls, which I don't think we'd do. Right? Or am I missing something here?
I mean, we have a G+ page and a facebook group, shouldn't those be enough?
The only reason I'm asking is because this would make it convenient / easy to promote HLP, thus hopefully gain more users, thus hopefully gain more modders.

However, if the only way to add buttons is to intrude into people's privacy, I can see your point. I'm pretty sure I've liked pages before that haven't asked for permissions though. *shrugs*
EDIT: If no button is possible, then how about just links to the FB and G+ pages?