The Free Software Foundation has released the GNU Affero General Public License version 3 (AGPLv3). Now if you build a web service using a AGPLv3 licensed software then you have to make your source code available, including your contribution to the code. Earlier, this restriction applied only to the ones who distributed the software. But this could be escaped by making the software available for usage through a network, like a service.
This seems to be very close to GPLv3, which means that we can expect similar objections. One good thing is that AGPLv3 is compatible with GPLv3, so you are allowed to club softwares licensed under these two licenses.
As a reminder, do not assume you do not have any restrictions when you use open source. As much as I hate it, and I am pathetic at it, ensuring that your work is compatible with the license of the software you are using is necessary.