Why is it worth $x? Simply put, because that's what they will be able to sell at. As to why that is, one could argue that it is in part because of a tech bubble

Why is it worth $x? Simply put, because that's what they will be able to sell at. As to why that is, one could argue that it is in part because of a tech bubble, but the most obvious and simple reason is indeed the rate of growth in their revenue and profit. I do disagree with your view that there is no benefit to being on LinkedIn. Personally, both my personal website (very low traffic bio site with a few blog posts) and twitter account are ranked above LinkedIn, and I like that LinkedIn is ranked third (that's for a search of my name on Google). I've never hired or been hired through LinkedIn, but people I want to work with (usually future clients) and people who consider working with me (again, clients) can look me up, and my LinkedIn page helps verify who I am, and what I do, they can see a few endorsements people have given me, and often they can see who they know that knows me. Also, after meeting someone at an event, a follow up LinkedIn reequest reminds them who you are and helps connect your face to your business card - and visa versa, helps you connect their face/job/etc to their card.