ResearchGate and Academia.edu are the two most popular social academic networks, or scholarly collaboration networks. They're useful for:
Typically, researchers in a particular field will be more active on one of these two networks, so if you are interested in joining one, it's probably a good idea to check out who's most active on them. In addition, you'll only be able to find and network with researchers who are active on the network, so it is not a great option for doing literature reviews or finding (all of) the latest research in your field.
It's also important to remember that these platforms are commercial social networks, similar to Facebook or Twitter, which means that they profit from the selling of the data you provide to them on the platform. They also encourage their users to upload their academic publications, which you very well may not have the legal right to do. For more details on these important caveats, please watch this ~3 minute video below.
This video, produced by the University of Rhode Island, mentions their Open Access policy and their institutional repository, a nonprofit platform which can be used by their community to make their scholarly work more globally visible and available to researchers, clinicians, taxpayers, alumni, and the public. Publishers usually have friendlier copyright policies towards nonprofit servers than towards commercial ones like ResearchGate and Academia.edu.
For faculty, staff, and students at Virginia Tech who wish to make their scholarly work available on a nonprofit server, they can do so via the institutional repository, VTechWorks, the data repository, VTechData; deposits to VTechWorks can also be made via Elements, the internal researcher profile and faculty activity reporting system (graduate students may also use this). Other nonprofit server options include disciplinary repositories, but some publishers' policies are not as lenient towards these repositories, because, presumably, these repositories are often the first place academics go for the latest research in their fields. However, VTechWorks and VTechData are indexed on major search engines and databases, such as Google Scholar, and such indexing dramatically boosts online discoverability of research; in fact, research has shown that self-archiving in such repositories (sometimes called "Green Open Access") leads to more views, downloads, and citations of scholarly works.
If you are interested in depositing a published, non-Open Access (subscription-access) work to a nonprofit repository and/or a commercial social academic network, you will need to first determine which version of your published academic work you can deposit to one or more of these platforms (if any). You can find this information via the publisher copyright policy database, SHERPA/RoMEO, or you can check your specific copyright publisher contract for more details.