No, but it would be naive to think that they are more likely to monitor individuals who frequent american gun forums than flower-arranging discussion groups.
Not to say that 'they' don't cross reference such information with plenty of other information.
Nothing new – I attended a meeting of the Anti-Nazi League in my first year of university, and after that all my mail was opened before I got it (before being carefully replaced and resealed with 'opened in error' or damaged in post' marked on it) for a year. One chap said I was imagining it – so as I was heading to the pigeon holes I said come with me – and lo! There was a ripped parcel carefully resealed into an outer envelope,
Shame, if I had gone to the 'correct' meeting i could have been signed up by MI5 
One day I'll see if I can get my file off them 
Neil (rejected enemy of the state)
(Just because you're paranoid it doesn't mean they aren't out to get you)