When you say "Online Social Networking"(OSN), these are the two main sites which strikes to our mind immediately, Orkut and Facebook. The main advantage of these OSN is, it facilitates people to be in touch at anytime/anywhere/anyone whom they want to. I can always see people preferring one over the other. Only those who're almost an OSN Addict, prefer both. So, what makes people choose one over the other? By "choose" i mean about "how active" they are in those OSN site. Here are few points, which i think 'mostly' decides or influences the preference of one over the other:
> Geographic Location: Though Orkut is popular in 'almost' all part of the world, even now we can see people in India having a certain Orkut appearance, before even they move to or even decide to tryout 'global' Facebook profile.
> Privacy: Though we decide to open ourselves in this world-wide-web, most people still prefer to close the doors, on certain details. This even extends to how your information is shared among other users in those communities. In this context, I believe Orkut offers more privacy options (which also limits its ability to be a 'complete' OSN) than the 'Complete' and 'Open' OSN Facebook.
> Target Audience: This basically depends on how you decide to share information, to whom to share and when to share. Information can be anything from what you're are doing currently (like 'i'm goin for a coffee') to Job Seeking/Vacancy details (in this recession time, i believe this is the most critical/essential information which you want to share/get). Whom to share refers to the group or number of persons/friends to whom you want those above said information to be shared/reached. When to share here refers to how frequently you want the information to be spread to all those whom you choose to share. So, If you want the whole people in your Friends List to get the information, very frequently, with an option of them to comment on your information, you will certainly go for Facebook than Orkut.
> Other Applications: In this completely stressful daily schedules, most people also want some entertainment (basically to time-pass) along with the OSN. Playing short and interesting games is one of the major time-pass people expect (as most people into this OSN are youth). In this quarter, Facebook definitely wins with total majority against Orkut (of course, Orkut also has applications, but i dont think not that popular as in FB). Now, Quizz-o-Mania is also spreading in Facebook like bacteria. This also includes, how much integration is allowed with other web 2.0 applications (like twitter,etc) or other sites (like flickr, etc) and the amount of customization that can be made.
The above points are not listed according to any importance. Each individual gives priority to one over the other point, which mostly decides on which one he/she chooses (choose - most active). Other than these points, i think all the other functionality/options are fairly same in both Orkut and Facebook. And as mentioned earlier, few OSN Addicts, choose both. Whatever OSN you choose, be sure to keep in touch with the people who adds value/meaning/happiness to your life. Have fun.