I'm suspicious of communities that grow too quickly.
1000 active members in the first week is not a good sign.
There is no history, no culture, no relationships between members. A community that grows quickly tends to dissipate quickly too. Novelty only carries a community so far.
Sure, there might be a handful of communities that witnessed and sustained rapid growth. These are the exceptions. It wont happen to you. Don't aim for this. Don't set these expectations.
Instead, you're going to have to work really hard. You're going to go through the community lifecycle. It will probably take you 3 - 9 months to get through the inception stage, 6 - 18 months to get through establishment and around 2 - 3 years to reach maturity.
That's a long time to be constantly pushing and developing the community. That's a long time to worry about it taking off. We've spoken to many organizations in recent months that are on the verge of killing their communities because they 'haven't taken off'.
Yet this wasn't true. These communities are exactly where they should be in the lifecycle. The problem wasn't progress, it was misplaced expectations. Too often, we aim for the quick win and ignore the process of building a community one active person at a time.
For a simple rule of thumb; if the numbers are gradually edging upwards, you're doing ok.
You can hope your community develops quicker than that, but it's better to plan and prepare for a long-term process. Misplaced expectations can be fatal.