Yeah I completely understand what you're saying Abomination, it seems almost useless because it's 1 character. You could just do this with JS or a simple conditional statement in the like.php (assuming that's the file).
I personally am not bothered about it, but I suppose pedantic people pick out these things and it would look better/professional. It's amazing what things people can judge things on.
@Disjunto, but then when there's 4 (standard 3 + 1 other) there's still going to be one other, unless you remove the 4th one when the 5th person likes it.
It's actually significantly more complicated and involved than you think, when you have to take into account how other languages form the plural—by no means is it as easy as adding an "s" to the word— and the fact that some languages have more than just singular and plural like English does. For example, there are also dual, trial, and paucal grammatical numbers in some languages. See http://en.wikipedia.org/wiki/Grammatical_number
Having said that, in this particular problem, we could apply a different solution: Edit: Just noticed that others hit on this solution as well.
A likes this.
A and B like this.
A, B, and C like this.
A, B, C, and D like this. <== just show the name of the 4th person. It will take the same space as saying "1 other" anyway.
A, B, C, and 2 others like this. <== with 5 or more, revert back to naming 3 people and saying "and x others".