- Used phone contacts from the mobile app to use as suggestions. I noticed a serious uptake on weird connection suggestions after I first installed the app a few years back. Checking it out on a proxy, I noticed that it wasn't using SSL, either, so I uninstalled that puppy quick. I assume they've fixed this between then and now.
- The "Find My Friends" from the Web and Mobile works both ways - so if someone you've emailed in the past chooses to do this, you start seeing them in your suggestions list. If I remember correctly, their original method for doing this was an actual login to email (which may be what happened to you; I don't know if there's any way to get them to clear out those suggestions once you've done it. I contacted Support about this awhile back & couldn't get an answer). What seemed weird to me about this is that all my wedding vendors popped up at the same time as suggestions...like they all got on LinkedIn at once or something.
- If you look at someone's profile or they look at yours--even anonymously, you're linked somehow for suggestions. I've seen people I haven't talked to or emailed in 15 years and three email accounts ago recommended, as well as their spouses(!). Serious privacy issues here if you spend a few minutes thinking about the possibilities. ;)
A quick tip--would bet many HN folks already know this, but I've talked to bunches of people who don't: Unless you explicitly set the "Select what others see when you view their profile," option (and keep checking it occasionally - just found mine was reset somehow), anyone whose profile you've viewed can see that you've viewed it. I love this feature (from the who's-viewed-me side, at least), but it's still creepy and something to be aware of if you're LinkedIn stalking someone...
Also, it does look like they finally added a "Security" checkbox in Settings to always browse using https - would highly recommend setting that one if you haven't already.
Of the two people I vividly remember as only vaguely familiar suggested people, neither was in my Gmail address book. I had a 2-sent-emails thread with one person, and no direct emails with the other. In 2007, Gmail might have already been auto-collecting contact info of people you email directly, but that can't explain the 2nd person.
I'm still hesitant to feel like it explains everything -- in 2007, LinkedIn still didn't seem that ubiquitous (after all, FB was then just opening itself up to non-college students, and LinkedIn's target demog. was still the pre-internet generation). So there weren't a whole lot of people I knew on the site to begin with, and there wasn't anybody I knew in common on LinkedIn connecting me to at least 2 of the vaguely familiar people I knew. And those 2 people started getting suggested to me by the site at the same time -- it seems too coincidental that these people (and others) provided their email contacts to LinkedIn at exactly the same time.
One good takeaway is, despite it taking me 5 more years to learn my lesson, and the really hard way :-(, I'm now using KeePass and creating difficult, unique passwords for each site, and using git to sync across computers. That helps.
So LinkedIn was snooping in my email, for which they would needed to have saved my password in plaintext (or equivalent) somewhere.
If I needed further proof, after my changing my email addr that I used to login to LinkedIn, I stopped getting those invasive people suggestions. I then googled (again, back in 2007) about LinkedIn and snooping in people's email accounts, and an interview with the CEO had a brief mention of it, where the CEO flatly denied any sort of privacy breach.
I think we need to supersede the commentary of LinkedIn's incompetent handling of security and privacy (which they spectacularly proved already) with the idea that not only do they not give a flip, but they want to keep it that way for their own maximum benefit.