Found a very good post on UAE Community regarding usage of Numeric digits in written Arabic. You'll find such writings on Blogs, Face Book or basically any online place where non-formal Arabic is acceptable. Some example words are

al7een, (alHeen)
9ya3a, (Sya'a)
3afana, ('afana)


Original Post
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Numbers are used as a substitute for various Arabic letters that either do not exist in English, or do not have the exact homophony.

2 = The strong accented (A) such as the one in Ahmad

3 = A deep throat sound, such as in (A) Al-Ain; the second (A).



5 = It is similar to the (Kh) in names like Khalid, Khalifa, Khalfan, etc.

6 = This is similar to an inflated and robust (T), such as Talal.

'6 = This is similar to its sibling; however, this exists in names like Al-Dahri in which the letter (D) resembles it the most.

7 = I pretty much guess that this is one of the most commonly used one, besides 3. This is that weird (H) in Ahmad.

8 = I don't know if you'll ever hear this letter, but Yemenis are notoriously known for using this letter. It's mostly replaced by a (G) by Emiratis, but for this one, you actually need a person who can say it to know it. It's similar to the letter (Q) in its truest English form.

9 = This is a bolded and inflated (S) in pronunciation. You can find this letter in such names as Saleh, Sultan, etc.