Many principal investigators have their publications listed on Google Scholar. If they do, one can easily determine how many times individual publications have been cited as well as the details of the citing articles. This is a great resource! Test it by typing in Brad Doble in the search field to link to Dr. Doble's profile...
A growing number of scientists use twitter to share information. Following the feeds of scientists with interests similar to your own can yield much useful knowledge. You can also learn a lot about a researcher's views on a wide spectrum of topics and gain some insight into their personalities.
ORCID stands for Open Researcher and Contributor ID. The ORCID is an alphanumeric code that uniquely identifies scientific and other academic authors and contributors. If you are a scientist, you should have one of these!
Linkedin is a networking site for all types of professionals, including those in academia and industry. Not all PIs have linkedin accounts, but many do, as do students, postdocs, and those who have ventured into science-related businesses. Here you can find much useful information regarding job opportunites, as well as scientific and technological advances, if you link to relevant accounts.
This is an essential site for finding biomedical publications. Doble lab members visit this site daily! One can also set up a National Center for Biotechnology Information (NCBI) user account to save searches that can be conducted automatically, with the results sent to you on a regular, even daily, basis.