Talking about filling a glass, I'd rather use "it" as possessive pronoun, not "her". Unless you are talking about filling up a gas tank.

Honestly it all comes down to laziness, whether they like to admit it or not. But there is more to it: "u" - For the most part, they are just being lazy and sloppy. Those who are meticulous don't write like that, unless they are using their smart phone to chat, in which case it makes it easier and faster to type. "wanna" - The majority of them sincerely believe that this informal contraction is "correct" speech. Chinese students get this idea from modern lyrics, poorly written blogs and text-based communication with either native English speakers or other foreigners who don't know better.