Unfortunately, it looks like there are no ready-made solutions for a "Like" button available for our older version of the forum software. Sooner or later, we will have to upgrade, the v1.1.x series won't be supported for ever.
SMF was chosen when our old YaBB forum became unstable and resource-intensive - due to the size of our forum then! - because it offered the simplest upgrade path, and had the same "look and feel". (In fact SMF was largely inspired by YaBB.)
But that was 2005 or something, and forums have come a long way since then. There are a variety of new platforms that offer all sorts of integration with social networks, image hosting, mobile support and a myriad of other features. If I was starting from scratch now, I almost certainly wouldn't choose SMF, I must admit. I'm very impressed with
Vanilla, and there are some subscription-based (ie cost money) forums out there that are quite highly regarded.
So when the time comes to upgrade this forum, we will probably look more widely at what's available.
Anyway, it doesn't look like a self-contained
Like button is going to happen in the near future.