Still new in the realm of research tools are social networking tools for researchers to connect. Academia.edu and Mendeley claim to offer new ways for researchers to find like-minded researchers, i.e. people with similar research interests.
academia.edu has often been described as a facebook for researchers. Its primary functions are to present oneself: Users associate themselves with a department of a university and add books and papers them have authored or read, talks they have given and their research interests to their profile. Currently academia.edu is listing about 112.000 users.
To view academia.edu in action, have a look at my profile.
The platform automatically informs me when people I am interested in (called "following") update their status or modify their research interests.
Users are able to browse academia.edu by research interests or by department. By these mechanisms you have a good chance to find like-minded researchers.
Mendeley serves a different purpose: 
Primarily it provides a software/UI to manage bibliographies, just like BibDesk (for LaTex), JabRef, or Firefox extension Zotero. However, additionally allows users to upload their bibliopgraphic data on their servers such that you can access the data online. And, implicitly, this provides as well for the possibiolity to share bibliography with other users.
Users may view each other, as in academia.edu, by profiles (to illustrate, have a look at my profile).
In comparison, I would venture, academia.edu fits better to browse through the realm of research institutions. Especially, academia.edu provides a more intutitive interaction with the myriad of research interests. Mendely, however, seems to offer much for existing groups who like to connect their bibliographies.
Although these promises are attractive, the economic and political question is: how to deal with the fact that both platforms are neither open source nor are the economic models transparent!
