I'm not a native English speaker, so I'm sorry if this is obvious but I can't find an explanation.
Why are "like"s usually referred to as like's
on Facebook? (You can see many instances here.)
To use a "word as a word" I would put quotes (if convenient using the apostrophe character in electronic text) around the word and place the plural -s outside them, and it seems that is the norm except on Facebook, e.g. "like"s.
By the way it seems the same applies to the "Discours particle" use of like, as @simchona says in the comments.
Best Answer
At http://englishplus.com/grammar/00000135.htm is found the following:
Lynne Truss also cites this usage in Eats, Shoots and Leaves.
(Unlike the first source above, Truss mentions the fact that there a few pure plurals allowed by some authorities to include apostrophes.)
Notice that these are talking about general, not Facebook-specific, English usages.
PS: My choice of like in the other thread is probably best regarded as arbitrary. Here is a parallel example: 'There are too many hamburger's in your essay.'