Subtle but finite difference you're (intentionally?) missing here: Monitoring public social media is not reaching into ("tapping") private conversations. When you post it on your Facebook you've lost any expectation of privacy. If you put it out there for all to see, someone is going to see it. Again, see Gibbs Rule 4.
Side bar: I recently had the chance to hear Carly Fiorina speak. She touched on this issue. She asked why US INS doesn't look at social media accounts of immigrants applying to come to the US. (They aren't allowed to, supposedly, per her.) She then pointed out when her granddaughter brings home a new boyfriend the first thing she does is find the lad's Facebook and Twitter account postings. So Carly does more social media vetting of people than US INS was her claim.
Now, as far as Trump being tapped -- he may not have been the target, but may have he been caught up in the tap anyway?
- Manafort was tapped:
http://www.cnn.com/2017/09/18/politics/paul-manafort-government-wiretapped-fisa-russians/index.html
- Manafort had an apartment in Trump Tower (was the monitoring happening to Manafort while there?)
- Manafort talked to Trump ... before and after the election
Conversations where Trump participated easily could've been monitored, but not with Trump as the prime target.
PS - That's not me; that's CNN. Enjoy the video and story at the above link.