I've not used it personally but the have heard good things about http://www.huddle.com/
What do you want your intranet platform to do?
- Collaboration - Spaces for documents - Access control - Used for forms? (surveys, polls etc? )
It's easier to suggest if there are more specific requirements.
Not sure about your requirements, but have heard that MS Sharepoint is suitable for almost everything.
With 1K employees, no one thing is going to provide best fit for everyone. Work out what the internet is for, and realise that you might end up using multiple systems for different uses, and find ways to use them together. That's a good thing, because it means you don't have a single monolithic companywide dependency.
Are you still a startup when you have 1000 employees?