I get far more quality recruiters contacting me that I ever have from sites like Monster, most of the ones on LinkedIn know the difference between Java and JavaScript and they don't take the one line in my history 5 years ago that said I helped build a printer script in C# that was used by a clients SAP system to mean I have 15 years SAP experience.
The value I get from it is mostly gathering intelligence about companies (e.g. size, new hires/leaves, key people in similar roles, etc), much less about actual "socializing". I very rarely initiate a connection, most of my (double digit only) contacts are by accepting invitations.