Is geliyom just a shorthand for "geliyorum" or does it have some other meaning?

 Yes. "Geliyom" is a shortened for "geliyorum".

In daily life some Turks shorten some verbs as its easy way to communicate !!!

like you brits shorten some words like " in it ?" instead of  " isn"t it "?

Thanks - but this brit doesn´t shorten things like that......
"isn´t it" is already a recognised shortened form of "is it not"

Yes it is. In fact all the grammatical persons can be shortened similarly in colloquial language.

Geliyorum > geliyom (I´m coming)

Geliyorsun > geliyon (You´re coming)

Geliyor > geliyo  (He/She/It´s coming)

Geliyoruz > geliyoz (We´re coming)

Geliyorsunuz > geliyonuz (You´re coming)

Geliyorlar > geliyolar (They´re coming)

With question particle mi, order of suffixes may change (it always comes as the final suffix with shorthand version):

Geliyor muyum? > geliyom mu? (Am I coming?)

Geliyor musun? > geliyon mu? (Are you coming)

Geliyor mu? > geliyo  mu? (Is he/she/it coming)

Geliyor muyuz? > geliyoz mu? (Are we coming)

Geliyor musunuz? > geliyonuz mu? (Are you coming)

Geliyorlar mı? > geliyolar mı? (Are they coming)

Of course during formal speech you should use the longer form.

 yeah  and you Turks say di mi ( di)

We shorten every possible word/sentence.

bi´ dâ´ki sefere hatırlı´ca´m ( bir dahaki sefere hatırlayacağım)

 Is "ain´t it?" more to your liking? I am from South London, after all

I dont think its write to compare "geliyom" with "isn´ t it" . Its more like "ya" instead of "you".

 Dear Zeytinne, we are talking about shortening the words,any words. Therefore any word that has been shortened is our subject. isn"t it or geliyom..

"I dunno what ya all talkin about" gibi bi´şey