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The noun chatmate designates a person with whom one chats, a partner in informal or friendly conversation.

In recent use, this noun has come to designate, more specifically, a person with whom one communicates via online chat or messaging. The following, for example, is from Up close but not too personal, by Asmaa Malik, published in the Ottawa Citizen (Ottawa, Ontario, Canada) of Monday 12^th April 2010 [Arts & Life section: page D2, column 2]:

The idea behind Chatroulette is impossibly simple. Meet random people face to face from the comfort of your living room.
[…]
The interface for the site […] consists merely of two webcam players and a chat dialogue box.
There are no usernames, no passwords, no friends’ lists. You see your potential, nameless chat-mates in the top player and your own image in the one below.

There is an isolated early use of the noun chatmate in the following from The Praise of the red Herring, in Nashes Lenten Stuffe, Containing, The Description and first Procreation and Increase of the towne of Great Yarmouth in Norffolke: With a new Play neuer played before, of the praise of the red Herring (London: Printed for N. L. and C. B., 1599), by the English poet, playwright, satirist and pamphleteer Thomas Nashe (1567-1601) [page 42]:

By the sea side on the other side stoode Heroes tower, such an other tower as one of our Irish castles, that is not so wide as a belfree, and a Cobler cannot iert out his elbowes in; a cage or pigeonhouse, romthsome enough to comprehend her and the toothlesse trotte her nurse, who was her onely chatmate and chambermaide.

This single use of chatmate by Thomas Nashe led some 19^th-century lexicographers to record this noun.—These are two examples:

1-: From A Supplementary English Glossary (London: George Bell and Sons, 1881), by Thomas Lewis Owen Davies [page 115, column 1]:

Cʜᴀᴛᴍᴀᴛᴇ, companion; one who chats with another.
The toothlesse trotte her nurse . . was her onely chatmate and chambermaide.—Nashe, Lenten Stuffe (Harl. Misc., vi. 167).

2-: From A New English Dictionary on Historical Principles; founded mainly on the materials collected by the Philological Society (Oxford: The Clarendon Press, 1893), edited by the British philologist and lexicographer James Augustus Henry Murray (1837-1915) [Vol. II: C, page 302, column 2]:

† Chatmate. Obs. rare. [f. Cʜᴀᴛ sb.¹ + Mᴀᴛᴇ.] A gossip.
1599 Nᴀꜱʜᴇ Lent. Stuffe (1871) 68 The toothless trot her nurse, who was her only chatmate and chambermaid.

These are, in chronological order, the next-earliest occurrences of the noun chatmate that I have found:

1-: From The Windlestraw (New York: Boni and Liveright Publishers, 1925), by the British novelist and historian John Mills Whitham (1883-1956) [chapter 9, page 71]:

He talked at considerable length of poor men and rich, saints and sinners, pleased by his responsive auditor.
“I like you,” he said, “you think about things—a good chatmate—you be!”

2-: From The Charlotte News (Charlotte, North Carolina, USA) of Sunday 11^th October 1931 [page 6, column 2]:

Police Salesmen.

What seems, to a person with a respect for efficiency, the greatest waste of time and energy in the governing of this City is the comparative lack of anything for the uptown traffic officers to do. All day, or at least for as much of the day as is bounded by their stretch of duty, they stand impressively, or loiter affably with some chatmate, or pass up and down chalking on the tires of parked automobiles a hieroglyphic that we doubt not is just as uninterpretable to them as it is to the private citizen.

3-: From the column ’Round Lima, Hour by Hour, by Oh. Oh. Jackenrim, published in The Lima News (Lima, Ohio, USA) of Wednesday 29^th April 1942 [page 6, column 3]:

Lunched with my wife, poor wretch, at a give full measure restaurant, so spread over the bill. Even took the potatoes on the menu, a new idea in personal service. Up the Main-st, saw Judge Neal Lora with some chat-mates and joined the bevy where Prosecutor Paul Landis (the former golfer and a good one) was in the throng.