We were going to add one (one exists), but Saylor makes a good point. Not because it'll annoy people during discussion threads, but because of the load. We're unfortunately a big site with not too much money, so we use shared hosting. A plugin like this sends off queries galore which equals server load. We're already on our last legs here and I don't think we can afford to upgrade to a VPS yet.
It is something that users would find useful here, but cool features are sacrificed for shared hosting I'm afraid

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Though, to end this post on a high, a developer on mybb.com is developing a notifications system which you all want. You'll get a notification similar to facebook when someone replies to your thread, quotes you in a thread, reps you etc. I'm also going to pay the developer to integrate it with our like system, so you get alerted when and who liked your post (if it's possible).