Do open-source foundations have oodles and piles of cash lying around for patent trolls to scoop up?
Are open-source softwares licensed under the GPL not immune to patent trolls since there is no profit motive?
One directly infringes a patent by making, selling, offering for sale, importing or using it. An open source foundation generally does not do these things. But it likely does encourage others to make, use, etc. technology that it essentially distributes. That can be an indirect infringement classed as contributory infringement or active inducement of infringement.
Fairly sure that the OSS "foundations" are all non-profit organisations that collect funds to pay for site hosting and put extra money into paying developers for their time, this would leave little for any troll to collect.
Note that GPLv3 section 11 specifies that a contributor grants a royalty-free patent license which should prevent a patent troll from suing.