In English, you like something. You is the subject, and something is the object.
However, in Spanish, the "thing being liked" is the subject and therefore the verb "gustar" is conjugated according to the thing being liked, pretty much like the verb "belong" in English:
In Spanish, the rationale is roughly (although incorrect):
- Esto gusta a mí -> Esto me gusta
- Ellas gustan a mí -> Ellas me gustan
The full correct expression is:
Gustar algo a alguien
- A mí me gusta algo
- A ti te gusta algo / A usted le gusta algo
- A él/ella le gusta algo
- A nosotros/nosotras nos gusta algo
- A vosotros/vosotras os gusta algo / A ustedes les gusta algo
- A ellos/ellas les gusta algo
It happens in German too that the thing being liked is the subject and the one who likes it is the (indirect) object (Mir = to me, dativ):
- Das gefällt mir = Mir gefällt das
Notice that "a mí = me", "a ti = te", etc... Yes, they are completely redundant, but that's how the expression is, and "a mi, a ti, etc..." are generally omitted.
Be careful with the Facebook sentence, though. It's incorrect probably because the one in charge of the translation algorithm didn't know proper Spanish and faced exactly the same problem you are describing.
In Spanish, the correct sentence would be:
You and 5 others like that -> A ti y a 5 personas más os/les gusta esto (os/les depending on regional differences for the plural of "tú").
Please also note that "ti" is never, ever used without a preposition before. It's always "A ti", "por ti", "de ti", etc...