Research and analysis.
When interpreting what a dormant user would be I assumed that they are likely to be in their late 20s - early 30s since most people graduate at 22 and spend a few years bouncing between a few companies to find the perfect job and when they find it they stay. They also could have stayed at their first job and began laying the foundation for their careers. Either way, after a few more years, in their late 20s and early 30s they are comfortable in their professional skills and begin to look for better positions.
I then interviewed three different people about their LinkedIn usage. I asked when the last time they used LinkedIn was, if it were longer than 3 months I asked why they didn't use LinkedIn, and I asked what would make them more interested in using LinkedIn again. On top of the basics I wanted to get a good view of expectations that they might not know they wanted so I asked what the most important aspect of their job. This made for some interesting results and allowed for more personalized results and content to keep users interested.
Everyone I asked except one person had not used LinkedIn in the last three months. One hadn't used it in two years because she was happy with her job, and the one who had used it in the last month was only to update her position. Before then, she had not used the site in months. One person told me that LinkedIn was "flat out too spammy" in reference to the constant pokes and prods to interact with colleagues as friends. This made it clear to me that these people were only interested in using the site if it was something important to their career or if it would help them find a job. When asked what the most important aspect of their job was the answers were in the range of about "flexibility" and "excitement." Understandable, since everyone I asked was a millennial. We expect a lot from our careers. There's a really interesting post on LinkedIn by Lisa Earle McLeod about it.