Adding another voice to the "do not like" camp.
For me the biggest issue is that a couple of people know my MN name; one close friend in a similar position to me, found me accidentally and let me know - I have no issues with that person knowing. The other went looking for me when she found out we were TTC/I mentioned MN and hasn't told me, but it's become clear from her knowing things I haven't told her. I don't particularly like her knowing, but not enough to namechange and lose all the MN friends I've made on conception threads etc. This person is exactly the sort of person who would use the like button to her FB though, especially if I said something she didn't like, or was pissed off at me and wanted to passive aggressively make a point. We have lots of shared FB friends and I have, in the past on conception and antenatal threads, given away quite a lot of personal info that RL friends/family could identify me with. Yes I know she could have already chosen to do this - but by putting a big (badly implemented) button at the top of every thread you're just making it far to easy for people like her.
Not to mention the fact that I regularly use MN from my iPad and it is far far too easy to accidentally press the "like" button myself (or indeed, for either of the two people mentioned above to do it by accident when reading one of my threads) - and given the fact that reports above say it posts to FB whether you give it permission or not - I could very easily out myself on FB to everyone I know. So I would have log out of MN to browse/read threads on iPad and only contribute when using the laptop, where it's easier to avoid accidentally pressing the like button. What a faff.
I've already gone and emptied my profile and made my (very few) photos MN friends only. I've reported a bunch of my old posts with the most identifying information on them and I can see a namechange coming on next time the main thread I post on gets full and a new one is started.
MN has been a godsend for me and provided some very real support during a difficult and stressful time. Sadly I can't see me giving away personal info in future as constantly namechanging to protect my identity is more effort than it's worth. But it's hard to make proper connections/support others in the same boat if you never give away anything real about yourself.
Not to mention - I had only recently started posting more on other threads, but will be back to lurking now.
I hate the ever encroaching reach of FB into our lives and really resent it being pushed ever harder on MN.