What is the value of those users? I'd argue they're basically worthless, and that this is a disastrously bad acquisition.
Let me propose some concrete measures: How often is the service's single sign-on login button used in your app? What is the value of the users in your app sourced from this service? I'd argue this is a great comparison, because if you can't monetize LinkedIn's users by sourcing them from LinkedIn traditionally (via ads, clever sourcing hacks, sign in buttons, etc.), you're certainly not going to monetize them effectively by owning the whole platform.
Where it is available, Facebook can be something like 10% of a free-to-play mobile game's logins. Facebook ads may be the source for as many as 20% of your users. Facebook login and Facebook sourced users often have long term value twice as high as everyone else.
Facebook is very valuable, and it's what made Zynga, Supercell, IAC (Tinder, Match.com) rich. Spotify might see something as high as 20% logins, and the diminished amount of account sharing alone must make them a few million dollars extra. Theoretically, if Facebook had not censored game post spam on its network, Zynga would still be a huge, rich company. If only Zynga could have bought Facebook instead of all those little gaming companies...
By comparison, LinkedIn retired their single sign in button. When it was available, it may have represented less than 1% of logins on a business service website and app. LinkedIn-sourced users had no different LTV, and represented less than 2% of all users.
LinkedIn was probably overvalued before Microsoft bought it, radically so. LinkedIn had no way of legitimately realizing the 20% fees recruiters enjoy—and even if they did, those fees exist exclusively in the tech world, exclusively for a narrow part of the workforce, with no credible path to introducing those fees to the rest of employment recruiting. Considering LinkedIn's poor track record overall of the value of those users, what is Microsoft going to do with them instead?
A bunch of companies try this path and fail. For example, GoDaddy is trying the same thing: Buy companies (you've never heard of) to try to cross-sell between GoDaddy users and company users. They make acquisitions based on incorrectly extrapolated LTVs for like 100 accounts they intensively hand cross-sold pre-acquisition. Then GoDaddy discovers two things a year later: the actual LTV of their test cohort, which is wildly below their estimate, and the actual value of the remaining 99,900 of their acquired users, which is nothing. Even if you bought a zero-revenue startup for $100,000 and your cost per acquired user (CPA) is $1.00, your LTV is still near $0, so what did you earn?
I suspect this purchase isn't about users. I suspect major LinkedIn investors shopped around the company and financial-engineered a sophisticated kickback for the decision makers on the acquirer's end. Much like when Google purchased the Google Ventures-backed Nest and enriched a huge number of insiders at great shareholder expense, the acquisition team at Microsoft was probably duped into enriching a great deal of insiders too.