On social networking sites, I tend to decline random friendship requests from people that I don't immediately recognize as acquaintances. If I have met them at any point in the past, however, and they seemed pleasant enough, I will accept the invitation as a means of coming to know that person better. To reject their request could easily be perceived as disinterest and cause them to abstain from future networking efforts with me. With that being said, I am not currently active on any social networking sites.
Oh, and as for bots, don't just reject the request. Report them.
