LinkedIn may be an online professional network, but that doesn't mean there are no other means of "transportation". I take care of my network by meeting my professional peers a lot. I talk with the same recruiters normally, both in a hiring capacity and a candidate capacity. I know them, they know me. I stay in touch with my old colleagues, some who are now CEOs, others who are programmers, and even more who are still where I met them - some of them I even consider friends - most of the valuable connections.
In the technology community, you can go to many organized events. I participate in both React, .NET, and cloud meetups, and now I co-host some as well.
Your network is so much more than LinkedIn, it's a relationship with actual human beings, which is where LinkedIn often falls short. It does, however, work great as a contact book of "oh shit, I wonder where he is now and how can I get in touch with him" - and then you go meet them. In person. All jobs of significance I have been considered for, and most likely most of what I will be considered for in the future is based on my actual relations with people. "Talk to this guy, I worked with him on X" - it's the same way I hire great people. LinkedIn is a showcase for your resume, what you've done, who you are affiliated with, and a professional point of contact, but it's by no means "your professional network".
In Germany there's XING which I think has a similar purpose (professional relationships, both for contractors as well as employees) and is relatively popular AFAICT. I'm not on XING, but what I heard about it from colleagues is that when IT contracting was at its height not so long ago they got several "cold" contacts per week from desperate people (MBA types, mostly) that have neither customers nor projects nor professional experience. Don't know if this is representative of XING; it might actually be not half-bad. But then the way the consulting business works here is that customers rarely approach freelancers directly and use agencies instead.