I think there are important differences between libraries and platforms.

1. Libraries are static, with versioning and upgrades, whereas platforms are real-time.

2. Libraries are based on a one-time usage, whereas platforms depend on providing constant value to maintain users.

3. Libraries do not generally offload any work to other servers, whereas platforms can provide efficiencies.

4. Libraries are platform, language, and version specific, whereas platforms can be built through protocols like REST and JSON that are known to most stacks.

That being said, open source libraries aren't always free when it comes to support and technical costs, and not all platforms are closed-source.