Maybe they are already here, and they just kinda blend, ya know? If I was an intergalactic alien robot tasked to conquer earth, I'd start my recon of earthlings by associating with the weak-minded parts of the population, because they would be less likely to notice discrepancies in my cover story while I improved by "camoflage" techniques.
Seems like NFL fan websites would be an ideal place to start - - they are a haven for the weak-minded, and require no initial face-to-face contact. Over time, my on-line interactions with humans would allow me to get better at avoiding obvious discrepancies in my cover story.
For example, I wouldn't want to blow my cover by claiming to be a Carolina farmer who listens to Howard Stern!
P.S. I've heard that the human eye is one of the more difficult anatomical features to replicate. I think that's why Harrison Ford used eye scanners to detect replicants in Blade Runner.