If you can shell out few bucks, reach out to them directly via phone and emails. Creating this database is crucial and may involve expert assistance.
LinkedIn is good to get traction but the process is slow.
I have worked on similar lines, so if you need any help, reach to me.
Best of luck.

Even though these are startups, I'm guessing that there's still an element of "guanxi" involved - i.e. the traditional personal connection dynamic of building business relationships in China.

If that is the case then just look at it as a slow burn. It can take months or even years for outsiders to properly build trust. As great as LinkedIn is, it doesn't necessarily factor in cultural differences.

I would also suggest trying to connect with some of the startup incubator communities there. I'm guessing that some of these startups are based in Shenzhen and I'm assuming there's a US Chamber of Commerce in the city (or any other major Chamber if you're not from the US). Reach out for them for guidance on the startup community. They'll have a better sense of the lay of the land there.