I had a spare couple of minutes and there was a new LJ release email in my inbox and so I found it for you by looking at previous release posts in lj_releases (which is worth watching for just these reasons).
The <lj-like> tag is mentioned in release #93 from June 19 2012.
The lj-like FAQ page.
LJ has a like button
Thu, 05:24: RT @ mcglk: I'm ridiculously excited about a reboot of B5. Normally, I wouldn't be. I mean, . . . well, as @ straczynski said: too…
Sun, 20:00: LOL! https://t.co/0KdX0y0KdQ
Sat, 14:10: .@ YodelOnline I just got a text saying that my parcel was left in my safe place and there is a card through my door. There is no…