As long as one doesn't use a non-AZERTY keyboard or a rather old cell phone (modern smartphones usually make the use of French diacritics easier than ever), there's no valid reasons to omit diacritics. I cannot talk for very young people and teenagers but, from my experience, people who are over twenty years old tend not to omit them, most of the time.
I would say that, in French, confusions mainly arise when diacritics alter the meaning of a word or a sentence rather than the pronunciation, so even in an informal conversation people would be better off not to omit them in this case if possible. This may be subjective, but I find it more confusing to read in a sentence
e.g. ou / a / du / arrive instead of où / à / dû / arrivé
e.g. eleve / tres / fete / naif / ca instead of élève / très / fête / naïf / ça.
That being said, I disagree with @jlliagre about the ç. In my opinion, omitting the cedilla is really not a big deal when it comes to understanding the meaning of a word. I've friends who don't bother with the ç when texting on their phone and replace it with a simple c instead. It has never confused me.
Though, if people who can't write the ç would replace every ç with one or a double s for phonetic reasons, we would expect to see, e.g. fasson / fransai / apersu instead of facon / francai / apercu, which is not the case, and is actually more confusing than anything else.
As for ça being written sa, this is a mistake made by people with poor grammar, who can't make the difference between the possessive determiner sa and the demonstrative determiner ça.
This awful mistake is so widely spread that we're sadly getting used to it, to the point where we are no longer confused by it… That doesn't mean that sa va should be less confusing than ca va. Quite the opposite I would say.