-Seeding: figure out a way to get data in there before the user experiences it. LinkedIn, Facebook, etc have you import contacts so you can instantly find your friends/colleagues/etc. Reddit used multiple accounts to post links themselves. Create the content or figure out a way to automatically populate when someone registers.
-Pre-Launch: Get a bunch of people interested and release it to a large number all at the same time. Setup a mailing list to collect email addresses and when it reaches whatever threshold you need, launch it.
-Launch at an event: depending on what it's used for, there could be an event which makes the most sense to launch at for people to experience it together. I think it was twitter that was launched at SXSW (my memory could be completely wrong about this).
-Launch and fail: probably the most likely outcome if you've not engaged your customers at all. You can try it and get feedback for the next attempt to launch it, you need to figure out why people would want to use it in the first place. AirBnb failed how many times?
At first it's a small party. Put everyone together. Only add stuff like "following" etc. later, when you have a big party that's too loud.