You've used the phrase "mental illness" to refer to autism a few times through this thread, but it's important to know that it's not really the right way to be describing it. An illness suggests something that is affecting the person and needs to be cured so they can get better. Autism is an inherent part of a person's personality, and not even necessarily disabling.
I have Asperger's. It's part of me and how I think. If you could somehow turn it off, I wouldn't be quite the same person any more.
That's essentially (at a child level, at least) the difference that used to be made between "autism" (the non-communicative type) and "Asperger's Syndrome" (better-functioning but doesn't get social skills), though classification has changed and it's all labeled autism now.
Asperger's people just don't naturally "get" social skills in the same way most people do. Things have to be explained to them, and they have to actively learn what others naturally pick up: Make eye contact. Ask people how they are. Don't interrupt people when they're talking, even if you really want to say something.
That's the important thing. Helping them learn how they need to behave, because they're not necessarily going to work it out for themselves.
I feel like the understanding of autism is shifting, and not necessarily in a good way. It's why I argued in that other (thoroughly more bitter) conversation that it's a bad thing that "autistic" is being used as a generalised insult - the real meaning of the term gets lost, and people start to think that it's a term for people who are antisocial and don't get along well with others. (And then possibly they self-diagnose as what they think is autistic, without really understanding it, and spread the impression further by telling everyone they've got it.)
Of course you can't go demanding proof that someone has clinically-diagnosed autism and hasn't just self-diagnosed off the Internet. But if someone just insists "I can't help being antisocial and not getting along with people because I have Asperger's"... that's not true at all. It makes it harder, but it's still up to you, your personality and how you choose to treat people. It just takes more active work.
Bonus question to others reading this: if you have Aspergers/autism, do you tell people? Personally I wouldn't ever bring it up unless it's relevant or I was directly asked. I'd rather stay quiet than make a big deal of it. Obviously I can't judge everyone's behaviour off mine, but I get a gut feeling that maybe the people who are eager to announce they have it might be the ones who don't really understand what it is.
Also, on realising whether people have it or not - when you're on an Internet forum you're going to be missing a lot of the cues that might mark someone as autistic: lack of eye contact, perhaps an odd way of speaking, talks at length on their favourite subject and misses your subtle clues that you're not interested... none of that applies here, and the awkwardness of social interaction doesn't carry over to carefully-thought-out text.
What sort of "small talk" are you trying to start with? Being asked to talk about myself to a stranger is awkward, and I'd most likely give minimal answers hoping you'll move onto something else - and I won't ask the same questions back to you, because (1) I feel funny asking people questions like that, especially when I'm not really interested in the answer, and (2) having asked that, I either have to remember it for next time, or risk looking rude because I asked and then didn't remember. (My mum asks those sorts of questions. I think she remembers everyone's answers in the same brain space that mine uses for memorising game information.)
Talk about something specific that interests them, and they might be more willing to engage. And don't expect them to want to have a one-on-one conversation "just because" - if this is within the game, and it's someone in your FC, just make sure they're included and have the chance to join in on conversations if they want. If it's a real-world friend, much the same applies - make them part of the group, but don't expect them to directly engage in conversation if they don't want to. Let them join in when they have something to say and feel comfortable doing so. (Don't avoid talking to them, just don't feel like you have to for the sake of filling up a silence.)
If the conversation is about something they're interested in or knowledgable about, they're likely to join in and contribute. They can be excellent sources of knowledge about the game, or whatever else is their topic of interest, and may be eager to help explain things to people if asked. They just need the right kind of opportunity to join in.