I had something like that happened to me at my previous gig where I was Head of Engineering (in Guadalajara, México).
We hired several good developers who were more or less jr. After an average of 2 years, 4 of them left to go to Google, Facebook, Amazon and some company in Austria.
For me it was an honor, a couple of them even told me that, they had been applying to those companies before but could not get in, and it was after they got into my team and got some specific experience, that these companies wanted to hire them.
In some other fields, it is expected that a Jr person will be for a couple of years and then "fly away". For example, I know it is a normal path for a just-graduated finance/economics person, who gets into a VC fund to get experience, and after 2 years he leaves for an MBA in a "top school". I've seen it several times (with people from VC funds that invested in companies I've been). That is something expected and accepted.
Personally I don't see anything bad for that. Maybe the only disadvantage is the "know how", but it can be mitigated if you establish the right documentation and knowledge transfer processes.