In terms of integrating your webapp with external webapps I see two different integration types: 

1. Inside-Out Integration: in this type external webapps can use services which are offered by your webapp. 
2. Outside-In Integration: this integration lets external webapps run in the context of your webapp. 

For more details on these two integration types and some examples please read my blog: 
Inside-Out and Outside-In Integration of Webapps: Services and Extensions 

The first integration type (inside-out) is more widespread than the second one (outside-in). 

In my opinion if you want your webapp to be successful, you need to think about both integration concepts directly in the beginning of your webapp development. Don't create a webapp without a concept of ecosystem anymore! 

What do you think? Do you have experiences and best practices doing especially the second type of integration (outside-in)?

Lofi Dewanto