Because it shows insincererity and annoyance.
It's as if you aren't sincere with "liking" their posts,
afterall, you're "liking" everyone's posts, you don't truely "like"
a post.
In my opinion, i don't really see that as a stalk, but as
mentioned before by another forumer, charlize, it may be a case
when it was "i broke up with him" and you happen to "like" it
unknowingly, you can't expect her not flare up. I mean, do you?
On the other hand, you may not have known her very well, if you
were doing that to your very and very good friends, i don't really
think they would mind. You said you met her on this sim sports
activity and added her, so as we can all see, both of you isn't so
close to each other.
I think TS, yourself, has to mature a bit and spare thoughts for
others before doing stuffs even though it may be out of jokes, or
even, "just for fun".
Just get her cell no and SMS her an apology, i'm sure she will
forgive you. It's just a small misunderstanding.