Everyone of my customers is gung ho about having his/her own social network. The customers think they have a better idea and can do a better job than the next guy. What they rarely consider though is that an even moderately successful community site is a lot of work. Some of the unknown requirements for any social network are listed here.
In order to keep your site successful you must pay attention to your community. You need to keep your users happy by addressing their needs and by continuously making your site better. A social network is no different from any other business from your customers' perspective. They expect service of some kind or another. The better the service, the more your users will turn to you instead of the next guy.
If you have 5,000 users, you will have a fairly steady flow of communication between them and you. A user might report a broken feature on your site. They may want to shoot the breeze with you. They may need to report another user. They may want to suggest a feature. You need to stay responsive to your users.
It is said that a happy customer will tell a few of his/her friends about the good experience he/she had with your company. An unhappy customer, on the other hand, will tell everyone they know about the bad experience they had with your company. Keep this in mind! The more the users, the more this becomes important.
If you have a lot of users, you (or someone else) will have to manage their activities. They will be adding content to your site on a regular basis. You will need to protect your users from inappropriate content. This means keeping an eye on all of the content in your site. Also as your users will be able to interact with one another through your site, you will need to ensure that there are features that at the very least allow users to protect themselves from other users. If you don't have this sort of feature then you will need a way for your users to report other users to you so that you can deal with it.
If you don't do this, you could end up with at least two problems. You might have a user uploading adult content. This content might offend some of your users. These users could easily take you to court and create havoc for you (even if your terms clearly say that you are not responsible...blah blah blah). The other possible problem is that if a user is offended, he/she may not come back to your site. This may not sound bad, but a social network is all about its users after all!
With any successful site—and not just social networks—it is very important that you keep your infrastructure two steps ahead of your users' needs. If your site is all about video feeds, then you will be required to keep a watchful eye on your bandwidth capabilities and disk space. If either of these starts to fail, the user experience of your site will start to diminish or cease altogether.
If your site has a large number of users regardless of your topic, you will need to watch your web server's usage. You may be required to have your site hosted on many servers. Or you may need to upgrade the overall robustness of your servers to support the heavier demand.
If you are not capable of infrastructure management, it is certainly well worth your will to find someone who can take care of this for you. If you only need part-time care, you might turn to someone like www.geeksontime.com. They provide on and offsite care for infrastructure administration. And don't forget to backup all your data. If this is also not your thing, turn to an automated service such as www.carbonite.com.