It depends on how old you are.
Being a teenager with a crush is fairly normal.
Going over the deep end with someone you only "talked to once on facebook" while in your 20s, 30s, 40s and beyond should be a cause worry.
Having said that there have been numerous couples who have carried on "emotional relationships" online as well as in times before there was the internet or telephones. Those were the days of pen pals and love letters.
As we mature we gain wisdom and learn to hold back emotionally until we truly get to know someone. The only way that really happens is by spending time together.