I think ICs are already in all the major social media back-ends that a nation hosts. I think that allies (like Israel and the US) regularly trade access to circumvent laws preventing surveillance of own citizens. Meanwhile LEOs of all stripes are already in social media, through the front-door.
The former is abhorrent and should be a big scandal, but it won't be. Snowden showed that Americans simply don't care. Given that signal, why stop?
The latter is actually good, in general. LEOs doing actual police work, joining groups that might be extremist...that's the online equivalent of undercover work, and I think it's good when it's human work.
So, basically I agree that proactive, general IC infiltration of back-ends is wrong, but I think its already happening and won't stop without change of sentiment. LEOs can and should do more human investigation, join groups, get involved, see what they are about - and then leave them alone if they aren't a physical threat. Obviously this can be super problematic (e.g. the FBI vs MLK) but the only alternative is to just sit there and wait for an attack. That's no bueno, either.