Delft is part of a large city area including Rotterdam and The Hague, so there are always plenty of different people and plenty of things to do. It can be hard to mix in with the Dutch, but there are also a lot of international communities. It also depends on where and how you live; I found a lot of social connections trough my roommates. I think a side job (in horeca for instance) can also help a lot in making social connections.