There seems to be a custom that men hug (cheek kiss) each other when they greet each other (especially if they did not see each other for a long time).   
Are Jews permitted to do so? (does it matter if they are single?)

The idea that it might be forbidden came from these sources:

Shulchan Aruch [20:1][1]
>הבא על אחת מן העריות דרך איברים או **שחבק ונשק** ונהנה בקירוב בשר הרי זה לוקה וחשוד על העריות:  
>...or he **hugged and kissed** (one of the people with whom sexual relations are forbidden) or had pleasure from the nearness of the flesh, he receives lashes....   

Shulchan Aruch [21:7][2]
>המחבק או המנשק אחת מהעריות שאין לבו של אדם נוקפו עליהם כגון אחותו הגדולה ואחות אביו וכיוצא בהם **אע"פ שאין לו שום הנאה כלל הרי זה מגונה ביותר ודבר איסור הוא ומעשה טפשים** שאין קרבים לערוה כלל בין גדול**ה** בין קטנ**ה** חוץ מהאב לבתו ומהאם לבנה   
>To hug or kiss one of the arayos whom people do not normally lust after, such as his adult sister or his aunt or similar, **even though he derives no pleasure at all, is very reprehensible, forbidden, and the act of a fool**. This is because we do not come close to arayos at all, whether (female) adult or minor, except the father to his daughter and the mother to her son.

Aruch Hashulchan [20:18][3]   
>כן אסור לאיש להתחכך לאיש באיבריו דרך תאוה   
>It is also forbidden for a man to rub another man on his limbs in a lustful way

Beer Haitiv [24:1][4]    
>surely a single man must stay away from to sleeping together (on the same bed) with another single man (in privacy), and this behavior should be protested against  


kissing might only be a problem if on the lips so this question is mainly regarding hugging   
Rabainu Bechaya [29:11][5]  (Ramban [29:9][6]) 
>(Regarding how was it permit for Yaakov to kiss Rochel) she was a child (under 3 years old) who can not have relations... **or we can say that this kiss was on the head or the shoulder, since the Scripture did not say "on her mouth"** ([Ibn Ezra][7])